/* ==========================================================================
   APPLE DESIGN SYSTEM & CUSTOM PROPERTIES
   ========================================================================== */

:root {
    /* Color Tokens - Apple Light Theme (Default) */
    --apple-bg: #f5f5f7;
    --apple-bg-secondary: #ffffff;
    --apple-card: #ffffff;
    --apple-card-hover: #ffffff;
    --apple-text: #1d1d1f;
    --apple-text-secondary: #86868b;
    --apple-text-tertiary: #a1a1a6;
    
    /* System Accents */
    --apple-blue: #0066cc;
    --apple-blue-rgb: 0, 102, 204;
    --apple-green: #30d158;
    --apple-red: #ff453a;
    --apple-orange: #ff9f0a;
    
    /* Structure & Accents */
    --apple-border: rgba(0, 0, 0, 0.08);
    --apple-divider: rgba(0, 0, 0, 0.06);
    --apple-shadow: 0 8px 30px rgba(0, 0, 0, 0.04);
    --apple-shadow-hover: 0 12px 40px rgba(0, 0, 0, 0.08);
    --apple-navbar-bg: rgba(245, 245, 247, 0.8);
    
    /* Theme Toggle support */
    --card-bg: var(--apple-card);
    
    /* Fonts */
    --font-system: "Plus Jakarta Sans", -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
    --font-heading: "Space Grotesk", -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
    
    /* Radii */
    --radius-sm: 8px;
    --radius-md: 12px;
    --radius-lg: 18px;
    --radius-xl: 22px;
    --radius-pill: 9999px;
    
    /* Animations & Transitions */
    --transition-apple: 0.4s cubic-bezier(0.25, 1, 0.5, 1);
    --transition-fast: 0.18s cubic-bezier(0.25, 1, 0.5, 1);

    /* Scrollbar Variables */
    --scrollbar-thumb: var(--apple-border);
    --scrollbar-track: transparent;
    scrollbar-color: var(--scrollbar-thumb) var(--scrollbar-track);
    scrollbar-width: thin;
}

/* Dark Mode Override */
body.dark-mode {
    --apple-bg: #000000;
    --apple-bg-secondary: #121212;
    --apple-card: #1d1d1f;
    --apple-card-hover: #2c2c2e;
    --apple-text: #f5f5f7;
    --apple-text-secondary: #86868b;
    --apple-text-tertiary: #48484a;
    
    --apple-blue: #2997ff;
    --apple-blue-rgb: 41, 151, 255;
    --apple-border: rgba(255, 255, 255, 0.08);
    --apple-divider: rgba(255, 255, 255, 0.06);
    --apple-shadow: 0 8px 30px rgba(0, 0, 0, 0.3);
    --apple-shadow-hover: 0 12px 45px rgba(0, 0, 0, 0.4);
    --apple-navbar-bg: rgba(0, 0, 0, 0.8);
    
    --card-bg: var(--apple-card);
}

/* High Contrast Mode for Scrollbars */
@media (prefers-contrast: more) {
    :root {
        --scrollbar-thumb: var(--apple-text);
        --scrollbar-track: var(--apple-bg-secondary);
    }
}

/* ==========================================================================
   RESET & FUNDAMENTALS
   ========================================================================== */
*, ::before, ::after {
    box-sizing: border-box;
}

body, h1, h2, h3, h4, p, ul, ol, li, canvas, input, select, button {
    margin: 0;
    padding: 0;
}

body {
    font-family: var(--font-system);
    background-color: var(--apple-bg);
    color: var(--apple-text);
    overflow-x: hidden;
    line-height: 1.47059;
    letter-spacing: -0.022em;
    min-height: 100vh;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
}

/* Smooth Theme Transitions */
body, 
.apple-navbar, 
.glass-card, 
.topic-card, 
.notes-container, 
.flashcard-front, 
.flashcard-back, 
.quiz-card, 
.btn, 
.nav-item, 
.tab-btn, 
.checkbox-container, 
.option-btn, 
select, 
.apple-input-container, 
.apple-input-container input, 
.checkmark, 
.note-box, 
.temperature-gauge, 
.graph-container, 
.canvas-container, 
.circular-score-container, 
.solution-display, 
.solution-formula-worked {
    transition: background-color var(--transition-apple), 
                color var(--transition-apple), 
                border-color var(--transition-apple), 
                box-shadow var(--transition-apple),
                scale var(--transition-apple),
                translate var(--transition-apple),
                opacity var(--transition-apple);
}

/* Selection Highlight customization */
::selection {
    background-color: color-mix(in oklab, var(--apple-blue), transparent 80%);
    color: var(--apple-text);
}

/* Standard scrollbar fallback wrapper */
@supports not (scrollbar-color: auto) {
    ::-webkit-scrollbar {
        width: 8px;
        height: 8px;
    }
    ::-webkit-scrollbar-track {
        background: transparent;
    }
    ::-webkit-scrollbar-thumb {
        background: var(--apple-border);
        border-radius: var(--radius-pill);
    }
    ::-webkit-scrollbar-thumb:hover {
        background: var(--apple-text-secondary);
    }
}

a, button {
    cursor: pointer;
}

button:focus-visible,
a:focus-visible {
    outline: 2px solid var(--apple-blue);
    outline-offset: 2px;
}

/* Typography elements */
h1, h2, h3, h4 {
    font-family: var(--font-heading);
    font-weight: 700;
    letter-spacing: -0.03em;
    color: var(--apple-text);
    text-wrap: balance;
}

p {
    color: var(--apple-text-secondary);
    font-size: 15px;
    line-height: 1.5;
    text-wrap: pretty;
}

code {
    font-family: SFMono-Regular, Consolas, "Liberation Mono", Menlo, monospace;
    font-size: 0.9em;
    padding: 3px 6px;
    background: var(--apple-border);
    border-radius: 4px;
    color: var(--apple-blue);
}

/* ==========================================================================
   LAYOUT STRUCTURE (APPLE HEADER NAVBAR + CONTAINER)
   ========================================================================== */

/* Top Navigation Bar */
.apple-navbar {
    position: sticky;
    top: 0;
    width: 100%;
    height: 48px;
    background-color: var(--apple-navbar-bg);
    backdrop-filter: blur(20px);
    -webkit-backdrop-filter: blur(20px);
    border-bottom: 1px solid var(--apple-border);
    z-index: 1000;
    display: flex;
    justify-content: center;
}

.navbar-inner {
    max-width: 1024px;
    width: 100%;
    height: 100%;
    padding: 0 22px;
    display: flex;
    align-items: center;
    justify-content: space-between;
}

.navbar-brand {
    display: flex;
    align-items: center;
    gap: 8px;
    font-weight: 600;
    font-size: 17px;
    letter-spacing: -0.015em;
    text-decoration: none;
    color: var(--apple-text);
}

.navbar-brand .logo-icon {
    font-size: 18px;
}

.navbar-menu {
    display: flex;
    align-items: center;
    gap: 28px;
}

.nav-item {
    position: relative;
    background: none;
    border: none;
    font-size: 12px;
    letter-spacing: -0.01em;
    color: var(--apple-text);
    opacity: 0.8;
    font-weight: 500;
    padding: 6px 0;
}

.nav-item::after {
    content: "";
    position: absolute;
    bottom: -2px;
    left: 0;
    width: 100%;
    height: 2px;
    background-color: var(--apple-blue);
    scale: 0 1;
    transform-origin: center;
    transition: scale var(--transition-apple);
}

.nav-item:hover {
    opacity: 1;
    color: var(--apple-blue);
}

.nav-item:hover::after {
    scale: 0.5 1;
}

.nav-item.active {
    opacity: 1;
    font-weight: 600;
    color: var(--apple-blue);
}

.nav-item.active::after {
    scale: 1 1;
}

.navbar-actions {
    display: flex;
    align-items: center;
    gap: 16px;
}

@media (max-width: 720px) {
    .apple-navbar {
        height: auto;
        min-height: 48px;
    }
    .navbar-inner {
        flex-direction: column;
        align-items: flex-start;
        height: auto;
        padding: 12px 16px 8px 16px;
        position: relative;
    }
    .navbar-brand {
        margin-bottom: 8px;
    }
    .navbar-menu {
        width: 100%;
        overflow-x: auto;
        flex-wrap: nowrap;
        gap: 16px;
        padding-bottom: 4px;
        -webkit-overflow-scrolling: touch;
        scrollbar-width: none;
    }
    .navbar-menu::-webkit-scrollbar {
        display: none;
    }
    .navbar-actions {
        position: absolute;
        top: 12px;
        right: 16px;
    }
}

.theme-toggle {
    background: none;
    border: none;
    font-size: 15px;
    padding: 4px;
    display: flex;
    align-items: center;
    justify-content: center;
    scale: 1;
}

.theme-toggle:hover {
    scale: 1.1;
}

.theme-toggle:active {
    scale: 0.95;
}

/* Master App Container */
.app-container {
    max-width: 1024px;
    width: 100%;
    margin: 0 auto;
    padding: clamp(16px, 4vw, 32px) clamp(12px, 3vw, 22px) 100px clamp(12px, 3vw, 22px);
}

.app-screen {
    display: none;
    animation: appleFadeUp 0.5s cubic-bezier(0.25, 1, 0.5, 1);
}

.app-screen.active {
    display: block;
}

@keyframes appleFadeUp {
    from {
        opacity: 0;
        translate: 0 12px;
    }
    to {
        opacity: 1;
        translate: 0 0;
    }
}

/* Page Intro Header Styles */
.screen-header {
    margin-bottom: clamp(24px, 5vw, 36px);
    text-align: center;
}

.screen-header h2 {
    font-size: clamp(28px, 5vw + 12px, 40px);
    line-height: 1.1;
    letter-spacing: -0.035em;
    margin-bottom: 10px;
}

.screen-header p {
    font-size: clamp(15px, 2vw + 10px, 19px);
    max-width: 600px;
    margin: 0 auto;
    color: var(--apple-text-secondary);
}

/* ==========================================================================
   GENERIC CARD & COMPONENT STYLES
   ========================================================================== */
.glass-card {
    background: var(--apple-card);
    border: 1px solid var(--apple-border);
    border-radius: var(--radius-lg);
    padding: clamp(18px, 4vw, 28px);
    box-shadow: var(--apple-shadow);
    transition: box-shadow var(--transition-apple), background-color var(--transition-apple), border-color var(--transition-apple);
}

.glass-card:hover {
    box-shadow: var(--apple-shadow-hover);
}

/* Pill buttons */
.btn {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    padding: 10px 22px;
    border-radius: var(--radius-pill);
    font-size: 14px;
    font-weight: 500;
    letter-spacing: -0.01em;
    border: 1px solid transparent;
    gap: 8px;
    scale: 1;
    transition: background-color var(--transition-apple), 
                color var(--transition-apple), 
                border-color var(--transition-apple), 
                box-shadow var(--transition-apple),
                scale var(--transition-apple);
}

.btn:hover {
    scale: 1.02;
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.08);
}

.btn:active {
    scale: 0.96;
}

.btn-primary {
    background-color: var(--apple-blue);
    color: #ffffff;
}

.btn-primary:hover {
    background-color: color-mix(in oklab, var(--apple-blue), var(--apple-bg) 15%);
}

.btn-secondary {
    background-color: transparent;
    border-color: var(--apple-border);
    color: var(--apple-text);
}

.btn-secondary:hover {
    background-color: var(--apple-border);
}

.btn-success {
    background-color: var(--apple-green);
    color: #ffffff;
}

.btn-success:hover {
    background-color: color-mix(in oklab, var(--apple-green), var(--apple-bg) 15%);
}

.btn-danger {
    background-color: var(--apple-red);
    color: #ffffff;
}

.btn-danger:hover {
    background-color: color-mix(in oklab, var(--apple-red), var(--apple-bg) 15%);
}

.btn-warning {
    background-color: var(--apple-orange);
    color: #ffffff;
}

.btn-warning:hover {
    background-color: color-mix(in oklab, var(--apple-orange), var(--apple-bg) 15%);
}

.w-full {
    width: 100%;
}

.hidden {
    display: none !important;
}

/* Badges */
.badge {
    font-size: 11px;
    font-weight: 600;
    padding: 3px 8px;
    border-radius: var(--radius-pill);
    letter-spacing: 0;
}

.badge-purple {
    background-color: rgba(138, 43, 226, 0.1);
    color: #a052ff;
}

.badge-cyan {
    background-color: rgba(0, 102, 204, 0.1);
    color: var(--apple-blue);
}

.badge-pink {
    background-color: rgba(255, 69, 58, 0.1);
    color: var(--apple-red);
}

/* Progress circle in header or wherever */
.global-stats-row {
    display: flex;
    align-items: center;
    gap: 20px;
    background: var(--apple-card);
    border: 1px solid var(--apple-border);
    border-radius: var(--radius-lg);
    padding: 20px;
    margin-bottom: 32px;
    box-shadow: var(--apple-shadow);
}

@media (max-width: 480px) {
    .global-stats-row {
        flex-direction: column-reverse;
        text-align: center;
        gap: 16px;
    }
}

.global-stats-text {
    flex-grow: 1;
}

.global-stats-text h3 {
    font-size: 22px;
    margin-bottom: 4px;
}

.global-progress-circle-wrap {
    width: 70px;
    height: 70px;
    position: relative;
    display: flex;
    justify-content: center;
    align-items: center;
}

.global-progress-circle-wrap svg {
    transform: rotate(-90deg);
}

.global-progress-circle-wrap .percent-text {
    position: absolute;
    font-size: 15px;
    font-weight: 700;
}

/* ==========================================================================
   SCREEN: STUDY HUB (DASHBOARD)
   ========================================================================== */
.welcome-banner {
    text-align: left;
    margin-bottom: clamp(24px, 5vw, 36px);
    padding-bottom: 24px;
    border-bottom: 1px solid var(--apple-divider);
}

.welcome-banner h2 {
    font-size: clamp(24px, 4vw + 12px, 34px);
    letter-spacing: -0.030em;
    margin-bottom: 8px;
}

.welcome-banner p {
    font-size: clamp(15px, 2vw + 10px, 17px);
    color: var(--apple-text-secondary);
}

.topics-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(min(100%, 300px), 1fr));
    gap: 24px;
}

.topic-card {
    background: var(--apple-card);
    border: 1px solid var(--apple-border);
    border-radius: var(--radius-lg);
    padding: 24px;
    box-shadow: var(--apple-shadow);
    display: flex;
    flex-direction: column;
    justify-content: space-between;
    min-height: 220px;
    translate: 0 0;
    transition: translate var(--transition-apple), box-shadow var(--transition-apple), background-color var(--transition-apple), border-color var(--transition-apple);
}

.topic-card:hover {
    translate: 0 -4px;
    box-shadow: var(--apple-shadow-hover);
}

.topic-card-header {
    display: flex;
    align-items: center;
    gap: 12px;
    margin-bottom: 12px;
}

.topic-num {
    font-size: 15px;
    font-weight: 600;
    color: var(--apple-blue);
    background: color-mix(in oklab, var(--apple-blue), transparent 90%);
    padding: 2px 8px;
    border-radius: var(--radius-pill);
}

.topic-card h3 {
    font-size: 19px;
    font-weight: 600;
}

.topic-desc {
    font-size: 14px;
    color: var(--apple-text-secondary);
    line-height: 1.45;
    margin-bottom: 20px;
}

.topic-footer {
    display: flex;
    justify-content: space-between;
    align-items: center;
    border-top: 1px solid var(--apple-divider);
    padding-top: 16px;
}

.progress-text {
    font-size: 13px;
    color: var(--apple-text-secondary);
    font-weight: 500;
}

/* Detailed Study Notes (Apple Developer Doc style) */
.notes-container {
    background: var(--apple-card);
    border: 1px solid var(--apple-border);
    border-radius: var(--radius-xl);
    padding: clamp(16px, 4vw, 40px);
    animation: appleFadeUp 0.4s ease-out;
}

.back-to-grid {
    margin-bottom: 30px;
}

.notes-content {
    max-width: 720px;
    margin: 0 auto;
    text-align: left;
}

.notes-content h2 {
    font-size: clamp(26px, 4vw + 12px, 34px);
    margin-bottom: 20px;
    letter-spacing: -0.035em;
}

.notes-content h3 {
    font-size: clamp(19px, 3vw + 10px, 22px);
    margin: 32px 0 14px 0;
    font-weight: 600;
}

.notes-content p {
    font-size: 16px;
    line-height: 1.6;
    margin-bottom: 18px;
    color: var(--apple-text);
}

.notes-content ul, .notes-content ol {
    margin-left: 20px;
    margin-bottom: 20px;
}

.notes-content li {
    font-size: 16px;
    line-height: 1.55;
    margin-bottom: 8px;
    color: var(--apple-text);
}

.notes-content li strong {
    font-weight: 600;
}

.note-box {
    background: var(--apple-bg);
    border-left: 3px solid var(--apple-blue);
    padding: 16px 20px;
    border-radius: var(--radius-md);
    margin: 24px 0;
}

.note-box strong {
    color: var(--apple-text);
}

/* ==========================================================================
   SCREEN: FLASHCARDS
   ========================================================================== */
.flashcard-game-container {
    max-width: 480px;
    width: 100%;
    margin: 20px auto;
    display: flex;
    flex-direction: column;
    gap: 24px;
}

.flashcard-progress {
    display: flex;
    align-items: center;
    gap: 16px;
}

.progress-bar-container {
    flex-grow: 1;
    height: 6px;
    background: var(--apple-border);
    border-radius: var(--radius-pill);
    overflow: hidden;
}

.progress-bar-fill {
    height: 100%;
    background-color: var(--apple-blue);
    border-radius: var(--radius-pill);
    transition: width var(--transition-apple);
}

.flashcard-counter {
    font-size: 12px;
    color: var(--apple-text-secondary);
    font-weight: 600;
}

/* 3D Physical Card container */
.flashcard-perspective {
    perspective: 1200px;
    width: 100%;
    height: 300px;
}

.flashcard-inner {
    width: 100%;
    height: 100%;
    position: relative;
    transform-style: preserve-3d;
    transform: rotateY(0deg);
    scale: 1;
    transition: transform var(--transition-apple), scale var(--transition-apple);
    cursor: pointer;
}

.flashcard-inner.flipped {
    transform: rotateY(180deg);
}

.flashcard-inner:hover {
    scale: 1.02;
}

.flashcard-inner:active {
    scale: 0.98;
}

.flashcard-front, .flashcard-back {
    position: absolute;
    width: 100%;
    height: 100%;
    backface-visibility: hidden;
    -webkit-backface-visibility: hidden;
    border-radius: var(--radius-xl);
    border: 1px solid var(--apple-border);
    padding: clamp(16px, 4vw, 32px);
    display: flex;
    flex-direction: column;
    justify-content: space-between;
    align-items: center;
    box-shadow: var(--apple-shadow);
    background: var(--apple-card);
    text-align: center;
    transition: background-color var(--transition-apple), border-color var(--transition-apple);
}

.flashcard-back {
    transform: rotateY(180deg);
}

.flashcard-category {
    font-size: 11px;
    text-transform: uppercase;
    font-weight: 700;
    letter-spacing: 0.05em;
    color: var(--apple-text-secondary);
}

.flashcard-question {
    font-size: clamp(18px, 3vw + 12px, 24px);
    font-weight: 700;
    line-height: 1.35;
    margin: auto 0;
    letter-spacing: -0.02em;
}

.flashcard-answer-container {
    margin: auto 0;
    display: flex;
    flex-direction: column;
    gap: 14px;
    align-items: center;
}

.flashcard-a {
    font-size: 16px;
    line-height: 1.5;
    color: var(--apple-text);
}

.flashcard-eq {
    font-size: 18px;
    font-weight: 600;
    background: color-mix(in oklab, var(--apple-blue), transparent 92%);
    color: var(--apple-blue);
    padding: 6px 14px;
    border-radius: var(--radius-md);
    display: inline-block;
    letter-spacing: -0.01em;
}

.flashcard-hint {
    font-size: 12px;
    color: var(--apple-text-tertiary);
}

.flashcard-controls {
    display: flex;
    gap: 12px;
    flex-wrap: wrap;
}

.flashcard-controls button {
    flex: 1 1 120px;
    min-width: 120px;
}

.flashcard-reset-container {
    background: var(--apple-card);
    border: 1px solid var(--apple-border);
    border-radius: var(--radius-xl);
    padding: clamp(20px, 6vw, 40px);
    text-align: center;
}

.flashcard-reset-container h3 {
    font-size: clamp(20px, 4vw + 10px, 24px);
    margin-bottom: 8px;
}

.flashcard-reset-container p {
    margin-bottom: 24px;
}

/* ==========================================================================
   SCREEN: SIMULATORS
   ========================================================================== */
.tab-selector {
    display: inline-flex;
    background: var(--apple-border);
    padding: 4px;
    border-radius: var(--radius-md);
    margin-bottom: 24px;
    align-self: center;
}

.tab-btn {
    background: transparent;
    border: none;
    padding: 8px 16px;
    font-weight: 600;
    font-size: 13px;
    color: var(--apple-text-secondary);
    border-radius: var(--radius-sm);
}

.tab-btn:hover {
    color: var(--apple-text);
}

.tab-btn.active {
    background-color: var(--apple-card);
    color: var(--apple-text);
    box-shadow: 0 2px 8px rgba(0,0,0,0.05);
}

.sim-panel {
    display: none;
    animation: appleFadeUp 0.4s ease-out;
}

.sim-panel.active {
    display: block;
}

.sim-grid {
    display: grid;
    grid-template-columns: 1.1fr 0.9fr;
    gap: 24px;
    align-items: start;
}

@media (max-width: 820px) {
    .sim-grid {
        grid-template-columns: 1fr;
    }
}

.sim-visualizer {
    display: flex;
    flex-direction: column;
    gap: 16px;
}

.sim-visualizer h3, .sim-readout h3 {
    font-size: 18px;
    font-weight: 600;
}

.canvas-container {
    background: #151517;
    border-radius: var(--radius-lg);
    border: 1px solid var(--apple-border);
    position: relative;
    overflow: hidden;
    display: flex;
    justify-content: center;
    align-items: center;
}

canvas {
    display: block;
    max-width: 100%;
    height: auto;
}

.sim-controls-row {
    display: flex;
    gap: 10px;
    margin-top: 4px;
}

.sim-readout {
    display: flex;
    flex-direction: column;
    gap: 20px;
    min-height: 100%;
}

.data-group {
    display: flex;
    flex-direction: column;
    gap: 12px;
}

.data-item {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding-bottom: 12px;
    border-bottom: 1px solid var(--apple-divider);
}

.data-item:last-child {
    border-bottom: none;
    padding-bottom: 0;
}

.data-item .label {
    font-size: 14px;
    color: var(--apple-text-secondary);
    font-weight: 500;
}

.data-item .value {
    font-size: 16px;
    font-weight: 600;
}

.graph-container {
    background: var(--apple-bg);
    border-radius: var(--radius-md);
    border: 1px solid var(--apple-border);
    padding: 12px;
}

.sim-desc {
    font-size: 13px;
    line-height: 1.45;
}

/* Custom CSS Sliders */
.slider-group {
    display: flex;
    flex-direction: column;
    gap: 16px;
    margin-top: 8px;
}

.slider-item {
    display: flex;
    flex-direction: column;
    gap: 6px;
}

.slider-item label {
    font-size: 13px;
    font-weight: 600;
    display: flex;
    justify-content: space-between;
}

.slider-item label span {
    color: var(--apple-blue);
}

/* Beautiful iOS Slider style */
input[type="range"] {
    -webkit-appearance: none;
    width: 100%;
    height: 4px;
    background: var(--apple-border);
    border-radius: var(--radius-pill);
    outline: none;
}

input[type="range"]::-webkit-slider-thumb {
    -webkit-appearance: none;
    appearance: none;
    width: 22px;
    height: 22px;
    border-radius: 50%;
    background: #ffffff;
    border: 0.5px solid rgba(0, 0, 0, 0.04);
    box-shadow: 0 3px 8px rgba(0,0,0,0.15);
    cursor: pointer;
    transition: transform var(--transition-fast);
}

input[type="range"]::-webkit-slider-thumb:hover {
    transform: scale(1.15);
}

input[type="range"]::-webkit-slider-thumb:active {
    transform: scale(0.95);
}

.temperature-gauge {
    display: flex;
    flex-direction: column;
    align-items: center;
    background: var(--apple-bg);
    border-radius: var(--radius-lg);
    padding: 18px;
    border: 1px solid var(--apple-border);
}

.temp-readout-big {
    font-size: 34px;
    font-weight: 700;
    line-height: 1.1;
    letter-spacing: -0.02em;
    color: var(--apple-text);
}

.temp-readout-c {
    font-size: 16px;
    color: var(--apple-text-secondary);
    font-weight: 500;
    margin-top: 2px;
}

/* ==========================================================================
   SCREEN: QUIZ MASTER
   ========================================================================== */
.quiz-container {
    max-width: 620px;
    margin: 0 auto;
}

.quiz-card {
    background: var(--apple-card);
    border: 1px solid var(--apple-border);
    border-radius: var(--radius-xl);
    padding: 36px;
    box-shadow: var(--apple-shadow);
}

.quiz-options-group {
    display: flex;
    flex-direction: column;
    gap: 12px;
    margin: 24px 0 30px 0;
}

/* Custom iOS Checklist */
.checkbox-container {
    display: flex;
    align-items: center;
    position: relative;
    padding: 14px 18px;
    background-color: var(--apple-bg);
    border-radius: var(--radius-md);
    font-size: 15px;
    font-weight: 500;
    cursor: pointer;
    border: 1px solid transparent;
    transition: all var(--transition-fast);
}

.checkbox-container:hover {
    background-color: var(--apple-border);
}

.checkbox-container input {
    position: absolute;
    opacity: 0;
    cursor: pointer;
}

.checkmark {
    width: 20px;
    height: 20px;
    border: 2px solid var(--apple-text-secondary);
    border-radius: 50%;
    margin-right: 14px;
    display: inline-flex;
    align-items: center;
    justify-content: center;
    transition: all var(--transition-fast);
    flex-shrink: 0;
}

.checkbox-container input:checked ~ .checkmark {
    border-color: var(--apple-blue);
    background-color: var(--apple-blue);
}

.checkbox-container input:checked ~ .checkmark:after {
    content: "✓";
    color: #ffffff;
    font-size: 13px;
    font-weight: 700;
}

.quiz-hud {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 8px;
}

.quiz-progress, .quiz-score {
    font-size: 13px;
    font-weight: 600;
    color: var(--apple-text-secondary);
}

.quiz-hud-progress-bar {
    height: 4px;
    background: var(--apple-border);
    border-radius: var(--radius-pill);
    overflow: hidden;
    margin-bottom: 24px;
}

.quiz-hud-progress-fill {
    height: 100%;
    background-color: var(--apple-blue);
    width: 0;
    transition: width var(--transition-apple);
}

.quiz-question-box {
    margin-bottom: 24px;
}

.quiz-tag {
    font-size: 11px;
    text-transform: uppercase;
    font-weight: 700;
    letter-spacing: 0.05em;
    color: var(--apple-blue);
    margin-bottom: 8px;
}

.quiz-question-box h3 {
    font-size: 21px;
    line-height: 1.4;
    letter-spacing: -0.02em;
    font-weight: 600;
}

.quiz-options {
    display: flex;
    flex-direction: column;
    gap: 12px;
    margin-bottom: 24px;
}

.option-btn {
    text-align: left;
    padding: 14px 20px;
    border-radius: var(--radius-md);
    background: var(--apple-bg);
    border: 1.5px solid transparent;
    font-size: 15px;
    color: var(--apple-text);
    font-weight: 500;
    line-height: 1.4;
    transition: all var(--transition-fast);
}

.option-btn:hover {
    background-color: var(--apple-border);
}

.option-btn.selected {
    border-color: var(--apple-blue);
    background-color: rgba(var(--apple-blue-rgb), 0.05);
}

.option-btn.correct-show {
    background-color: rgba(48, 209, 88, 0.1);
    border-color: var(--apple-green);
    color: var(--apple-text);
}

.option-btn.incorrect-show {
    background-color: rgba(255, 69, 58, 0.1);
    border-color: var(--apple-red);
}

.quiz-feedback-box {
    border-radius: var(--radius-md);
    padding: 18px;
    margin-bottom: 24px;
    animation: appleFadeUp 0.3s ease-out;
}

.quiz-feedback-box.correct {
    background-color: rgba(48, 209, 88, 0.08);
    border: 1px solid rgba(48, 209, 88, 0.15);
}

.quiz-feedback-box.incorrect {
    background-color: rgba(255, 69, 58, 0.08);
    border: 1px solid rgba(255, 69, 58, 0.15);
}

.feedback-title {
    font-size: 15px;
    font-weight: 700;
    margin-bottom: 4px;
}

.quiz-feedback-box.correct .feedback-title { color: var(--apple-green); }
.quiz-feedback-box.incorrect .feedback-title { color: var(--apple-red); }

.feedback-text {
    font-size: 14px;
    line-height: 1.45;
    color: var(--apple-text);
}

.quiz-actions {
    display: flex;
    gap: 12px;
    justify-content: flex-end;
}

.circular-score-container {
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    background: var(--apple-bg);
    width: 140px;
    height: 140px;
    border-radius: 50%;
    margin: 30px auto;
    border: 1px solid var(--apple-border);
}

#results-score-big {
    font-size: 38px;
    font-weight: 800;
    color: var(--apple-blue);
    line-height: 1;
}

.results-summary p {
    font-size: 16px;
    margin-bottom: 24px;
    text-align: center;
}

.results-actions {
    display: flex;
    gap: 12px;
    justify-content: center;
}

/* ==========================================================================
   SCREEN: EQUATION SOLVER
   ========================================================================== */
.solver-grid {
    display: grid;
    grid-template-columns: 1.15fr 0.85fr;
    gap: 24px;
    align-items: start;
}

@media (max-width: 820px) {
    .solver-grid {
        grid-template-columns: 1fr;
    }
}

.solver-config {
    display: flex;
    flex-direction: column;
    gap: 20px;
}

/* Apple Dropdown/Select field style */
.select-wrapper {
    position: relative;
    width: 100%;
}

select {
    width: 100%;
    padding: 12px 16px;
    border-radius: var(--radius-md);
    background-color: var(--apple-bg);
    border: 1px solid var(--apple-border);
    font-size: 15px;
    font-weight: 500;
    color: var(--apple-text);
    outline: none;
    -webkit-appearance: none;
    appearance: none;
    transition: all var(--transition-fast);
}

select:focus {
    border-color: var(--apple-blue);
    box-shadow: 0 0 0 3px rgba(var(--apple-blue-rgb), 0.15);
}

.select-wrapper::after {
    content: "▼";
    font-size: 10px;
    color: var(--apple-text-secondary);
    position: absolute;
    right: 16px;
    top: 50%;
    transform: translateY(-50%);
    pointer-events: none;
}

.solver-formula-display {
    background: var(--apple-bg);
    border-radius: var(--radius-md);
    padding: 18px;
    text-align: center;
    border: 1px solid var(--apple-border);
}

.solver-formula-display .formula-latex {
    font-size: 26px;
    font-weight: 700;
    background: none;
    color: var(--apple-text);
    letter-spacing: -0.02em;
}

.solver-formula-display p {
    font-size: 13px;
    margin-top: 6px;
}

.solver-inputs {
    display: flex;
    flex-direction: column;
    gap: 14px;
}

.input-field-wrap {
    display: flex;
    flex-direction: column;
    gap: 6px;
}

.input-field-wrap label {
    font-size: 13px;
    font-weight: 600;
}

/* Apple input fields */
.apple-input-container {
    display: flex;
    background-color: var(--apple-bg);
    border: 1px solid var(--apple-border);
    border-radius: var(--radius-md);
    overflow: hidden;
    align-items: center;
    padding-right: 14px;
}

.apple-input-container:focus-within {
    border-color: var(--apple-blue);
    box-shadow: 0 0 0 3px rgba(var(--apple-blue-rgb), 0.15);
}

.apple-input-container input {
    flex-grow: 1;
    border: none;
    background: transparent;
    padding: 12px 16px;
    font-size: 15px;
    color: var(--apple-text);
    outline: none;
}

.apple-input-container .unit {
    font-size: 13px;
    font-weight: 600;
    color: var(--apple-text-secondary);
}

.solution-display {
    background: var(--apple-bg);
    border-radius: var(--radius-md);
    padding: 24px;
    border: 1px solid var(--apple-border);
    margin-bottom: 20px;
    text-align: center;
}

.solution-label {
    font-size: 13px;
    font-weight: 600;
    color: var(--apple-text-secondary);
    text-transform: uppercase;
    letter-spacing: 0.05em;
    margin-bottom: 8px;
}

.solution-value {
    font-size: 36px;
    font-weight: 800;
    color: var(--apple-blue);
    letter-spacing: -0.02em;
    margin-bottom: 12px;
}

.solution-formula-worked {
    font-size: 14px;
    color: var(--apple-text);
    line-height: 1.6;
    background: var(--apple-card);
    padding: 20px;
    border-radius: var(--radius-md);
    border: 1px solid var(--apple-border);
    text-align: left;
    overflow-x: auto;
    margin-top: 16px;
    box-shadow: 0 2px 10px rgba(0,0,0,0.03);
}

.solver-var-unit-select {
    border: none;
    background: var(--apple-border);
    border-radius: var(--radius-sm);
    font-size: 12px;
    font-weight: 600;
    color: var(--apple-text);
    cursor: pointer;
    padding: 4px 8px;
    outline: none;
    width: auto;
    transition: background-color var(--transition-fast);
}

.solver-var-unit-select:hover {
    background-color: var(--apple-divider);
}

.solution-primary-val {
    font-size: 32px;
    font-weight: 800;
    color: var(--apple-blue);
    margin-bottom: 16px;
}

.solution-conversions {
    margin-top: 12px;
    padding-top: 12px;
    border-top: 1px solid var(--apple-divider);
    font-size: 14px;
    color: var(--apple-text-secondary);
}

.conversion-label {
    display: block;
    font-weight: 600;
    margin-bottom: 6px;
    text-transform: uppercase;
    font-size: 11px;
    letter-spacing: 0.05em;
}

.conversion-items {
    font-weight: 500;
    color: var(--apple-text);
}

.formula-guide-box h3 {
    font-size: 14px;
    font-weight: 700;
    margin-bottom: 10px;
}

.variable-list {
    list-style: none;
    display: flex;
    flex-direction: column;
    gap: 8px;
}

.variable-list li {
    font-size: 13px;
    color: var(--apple-text-secondary);
    line-height: 1.4;
}

.variable-list li strong {
    color: var(--apple-text);
    font-weight: 600;
}

/* ==========================================================================
   ACCESSIBILITY & SEMANTIC ENHANCEMENTS
   ========================================================================== */

/* High-contrast focus indicator for keyboard users */
:focus-visible {
    outline: 2px solid var(--apple-blue);
    outline-offset: 2px;
}

/* Specific styling for circular elements focus outline */
.theme-toggle:focus-visible {
    border-color: var(--apple-blue);
    box-shadow: 0 0 0 3px rgba(var(--apple-blue-rgb), 0.3);
    outline: none;
}

/* Hides content visually but keeps it in the accessibility tree */
.visually-hidden:where(:not(:focus-within, :active)) {
    position: absolute !important;
    clip-path: inset(50%) !important;
    overflow: hidden !important;
    width: 1px !important;
    height: 1px !important;
    margin: -1px !important;
    padding: 0 !important;
    border: 0 !important;
    white-space: nowrap !important;
}

/* Skip link positioning and style */
.skip-link {
    background-color: var(--apple-blue);
    color: #ffffff;
    padding: 10px 20px;
    position: absolute;
    top: 12px;
    left: 22px;
    z-index: 9999;
    border-radius: var(--radius-md);
    text-decoration: none;
    font-size: 14px;
    font-weight: 600;
    box-shadow: var(--apple-shadow-hover);
    transition: transform var(--transition-fast);
}

.skip-link:focus-visible {
    outline: 2px solid #ffffff;
    outline-offset: -4px;
}

/* Semantic list inside the navbar menu */
.navbar-menu ul {
    display: flex;
    align-items: center;
    gap: 28px;
    list-style: none;
    padding: 0;
    margin: 0;
}