:root {
    --primary-color: #2563eb;
    --primary-dark: #1d4ed8;
    --primary-light: #3b82f6;
    --secondary-color: #64748b;
    --accent-color: #06b6d4;
    --success-color: #10b981;
    --warning-color: #f59e0b;
    --danger-color: #ef4444;

    /* Light theme colors */
    --bg-primary: #ffffff;
    --bg-secondary: #f8fafc;
    --bg-tertiary: #f1f5f9;
    --text-primary: #1e293b;
    --text-secondary: #64748b;
    --text-muted: #94a3b8;
    --border-color: #e2e8f0;
    --border-hover: #cbd5e1;
    --card-bg: #ffffff;
    --card-shadow: rgba(0, 0, 0, 0.1);
    --header-bg: #ffffff;

    /* Gradients for light theme */
    --gradient-primary: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
    --gradient-secondary: linear-gradient(135deg, #f093fb 0%, #f5576c 100%);
    --gradient-tertiary: linear-gradient(135deg, #4facfe 0%, #00f2fe 100%);
    --gradient-quaternary: linear-gradient(135deg, #43e97b 0%, #38f9d7 100%);
    --gradient-modern: linear-gradient(135deg, #ff9a9e 0%, #fecfef 100%);

    --shadow-sm: 0 1px 2px 0 rgba(0, 0, 0, 0.05);
    --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px rgba(0, 0, 0, 0.06);
    --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.1), 0 4px 6px -2px rgba(0, 0, 0, 0.05);
    --border-radius: 12px;
    --transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);

    /* Typography scale */
    --font-size-xs: 0.8125rem;
    --font-size-sm: 0.9rem;
    --font-size-base: 1rem;
    --font-size-lg: 1.125rem;
    --font-size-xl: 1.25rem;
    --font-size-2xl: clamp(1.5rem, 3vw, 2rem);
    --font-size-3xl: clamp(2rem, 4vw, 2.5rem);
    --font-size-4xl: clamp(2.5rem, 5vw, 3rem);
}

[data-theme="dark"] {
    /* Dark theme colors */
    --bg-primary: #0f172a;
    --bg-secondary: #1e293b;
    --bg-tertiary: #334155;
    --text-primary: #f1f5f9;
    --text-secondary: #cbd5e1;
    --text-muted: #94a3b8;
    --border-color: #334155;
    --border-hover: #475569;
    --card-bg: #1e293b;
    --card-shadow: rgba(0, 0, 0, 0.3);
    --header-bg: #0f172a;

    /* Darker gradients for dark theme */
    --gradient-primary: linear-gradient(135deg, #4338ca 0%, #6366f1 100%);
    --gradient-secondary: linear-gradient(135deg, #ec4899 0%, #f43f5e 100%);
    --gradient-tertiary: linear-gradient(135deg, #0ea5e9 0%, #06b6d4 100%);
    --gradient-quaternary: linear-gradient(135deg, #059669 0%, #10b981 100%);
    --gradient-modern: linear-gradient(135deg, #e11d48 0%, #be185d 100%);
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    transition: background-color 0.3s ease, color 0.3s ease, border-color 0.3s ease;
}

body {
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
    line-height: 1.6;
    color: var(--text-primary);
    background: var(--bg-primary);
    min-height: 100vh;
    font-size: var(--font-size-base);
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
}

.container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 1.5rem;
}

/* --- Base Typography --- */
h1, h2, h3, h4, h5, h6 {
    color: var(--text-primary);
    line-height: 1.25;
    letter-spacing: -0.01em;
}
h1 { font-size: var(--font-size-4xl); font-weight: 800; }
h2 { font-size: var(--font-size-3xl); font-weight: 700; }
h3 { font-size: var(--font-size-2xl); font-weight: 700; }
h4 { font-size: var(--font-size-xl); font-weight: 600; }
h5 { font-size: var(--font-size-lg); font-weight: 600; }
h6 { font-size: var(--font-size-base); font-weight: 600; color: var(--text-secondary); }

p { margin: 0 0 1rem; color: var(--text-secondary); }
ul, ol { margin: 0 0 1.1rem 1.25rem; color: var(--text-secondary); }
li { margin: 0.35rem 0; }

a { color: var(--primary-color); text-underline-offset: 3px; text-decoration-thickness: 2px; }
a:hover { color: var(--primary-dark); }
a:focus { outline: none; box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.35); border-radius: 4px; }

hr { border: 0; border-top: 1px solid var(--border-color); margin: 2rem 0; }

/* (Removed legacy theme toggle styles) */

/* Topics Dropdown */
.dropdown {
    position: relative;
}

.dropdown-toggle {
    cursor: pointer;
}

.dropdown-menu {
    position: absolute;
    top: 100%;
    left: 0;
    background: var(--card-bg);
    border: 1px solid var(--border-color);
    border-radius: var(--border-radius);
    box-shadow: var(--shadow-lg);
    padding: 0.5rem 0;
    min-width: 180px;
    opacity: 0;
    visibility: hidden;
    transform: translateY(-10px);
    transition: all 0.3s ease;
    z-index: 1000;
}

.dropdown:hover .dropdown-menu,
.dropdown:focus-within .dropdown-menu {
    opacity: 1;
    visibility: visible;
    transform: translateY(0);
}

.dropdown-menu li {
    list-style: none;
}

.dropdown-menu a {
    display: block;
    padding: 0.5rem 1rem;
    color: var(--text-secondary);
    text-decoration: none;
    transition: all 0.2s ease;
}

.dropdown-menu a:hover {
    background: var(--bg-secondary);
    color: var(--primary-color);
}

/* Key Ideas and Common Mistakes Boxes */
.key-idea-box, .common-mistake-box {
    padding: 1.5rem;
    border-radius: var(--border-radius);
    margin: 2rem 0;
    position: relative;
    border-left: 4px solid;
}

.key-idea-box {
    background: linear-gradient(135deg, rgba(16, 185, 129, 0.05), rgba(16, 185, 129, 0.1));
    border-left-color: var(--success-color);
}

.common-mistake-box {
    background: linear-gradient(135deg, rgba(239, 68, 68, 0.05), rgba(239, 68, 68, 0.1));
    border-left-color: var(--danger-color);
}

.key-idea-box::before {
    content: '💡 Key Idea';
    position: absolute;
    top: -10px;
    left: 20px;
    background: var(--card-bg);
    padding: 0 10px;
    font-weight: 600;
    color: var(--success-color);
    font-size: 0.9rem;
}

.common-mistake-box::before {
    content: '⚠️ Common Mistake';
    position: absolute;
    top: -10px;
    left: 20px;
    background: var(--card-bg);
    padding: 0 10px;
    font-weight: 600;
    color: var(--danger-color);
    font-size: 0.9rem;
}

/* --- Navigation --- */
.nav {
    background: var(--card-bg);
    border-bottom: 1px solid var(--border-color);
    padding: 1rem 0;
    position: sticky;
    top: 0;
    z-index: 100;
    backdrop-filter: blur(10px);
}

.nav-content {
    display: flex;
    align-items: center;
    justify-content: space-between;
}

.nav-logo {
    font-size: 1.5rem;
    font-weight: 700;
    color: var(--primary-color);
    text-decoration: none;
}

.nav-links {
    display: flex;
    gap: 2rem;
    list-style: none;
}

.nav-links a {
    color: var(--text-secondary);
    text-decoration: none;
    font-weight: 500;
    transition: var(--transition);
}

.nav-links a:hover,
.nav-links a.active {
    color: var(--primary-color);
}

/* Mobile Navigation (Hamburger Menu) */
.nav-toggle {
    display: none;
    background: none;
    border: none;
    cursor: pointer;
    padding: 0.5rem;
    flex-direction: column;
    gap: 3px;
}

.nav-toggle span {
    width: 20px;
    height: 2px;
    background: var(--text-primary);
    transition: var(--transition);
}

.nav-toggle.active span:nth-child(1) {
    transform: rotate(45deg) translate(5px, 5px);
}

.nav-toggle.active span:nth-child(2) {
    opacity: 0;
}

.nav-toggle.active span:nth-child(3) {
    transform: rotate(-45deg) translate(7px, -6px);
}

/* --- Back to Top Button --- */
.back-to-top {
    position: fixed;
    right: 1.5rem;
    bottom: 1.5rem;
    display: none;
    align-items: center;
    justify-content: center;
    width: 44px;
    height: 44px;
    border-radius: 999px;
    border: 1px solid var(--border-color);
    background: var(--card-bg);
    color: var(--text-primary);
    box-shadow: var(--shadow-md);
    cursor: pointer;
    transition: var(--transition);
    z-index: 999;
}
.back-to-top:hover { transform: translateY(-2px); border-color: var(--border-hover); }
.back-to-top.visible { display: inline-flex; }

/* --- Breadcrumb --- */
.breadcrumb {
    background: var(--bg-secondary);
    padding: 1rem 0;
    border-bottom: 1px solid var(--border-color);
}
.breadcrumb nav { display: flex; align-items: center; gap: 0.5rem; font-size: 0.9rem; }
.breadcrumb a { color: var(--text-secondary); text-decoration: none; transition: var(--transition); }
.breadcrumb a:hover { color: var(--primary-color); }
.breadcrumb .separator { color: var(--text-muted); }
.breadcrumb .current { color: var(--text-primary); font-weight: 500; }

/* --- Header --- */
.header {
    background: var(--bg-secondary);
    padding: 4rem 0 3rem;
    text-align: center;
    border-bottom: 1px solid var(--border-color);
}
.header h1 { font-size: clamp(2rem, 4vw, 3rem); font-weight: 700; color: var(--text-primary); margin-bottom: 1rem; }
.header p, .header .subtitle, .header .description { font-size: 1.2rem; color: var(--text-secondary); max-width: 700px; margin: 0 auto; }

/* --- Footer --- */
.footer {
    text-align: center;
    padding: 3rem 0;
    background: var(--card-bg);
    border-top: 1px solid var(--border-color);
    margin-top: 4rem;
}
.footer p { color: var(--text-secondary); font-size: 0.95rem; }
.footer-links { display: flex; justify-content: center; gap: 2rem; margin-bottom: 1rem; flex-wrap: wrap; }
.footer-links a { color: var(--text-secondary); text-decoration: none; font-weight: 500; transition: var(--transition); }
.footer-links a:hover { color: var(--primary-color); }

/* --- Chapter and Progress Styles --- */
.objectives-section {
    padding: 3rem 0;
    background: var(--bg-primary);
}

.objectives-card {
    background: var(--card-bg);
    border-radius: var(--border-radius);
    padding: 2rem;
    border: 1px solid var(--border-color);
    box-shadow: var(--shadow-sm);
    transition: var(--transition);
}
.objectives-card:hover { box-shadow: var(--shadow-md); transform: translateY(-2px); }

.objectives-card h2 {
    font-size: 1.5rem;
    color: var(--text-primary);
    margin-bottom: 2rem;
    text-align: center;
}

.objectives-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
    gap: 1.5rem;
}

.objective-item {
    text-align: center;
    padding: 1rem;
}

.objective-icon {
    font-size: 2rem;
    display: block;
    margin-bottom: 1rem;
}

.objective-item h3 {
    font-size: 1.1rem;
    color: var(--text-primary);
    margin-bottom: 0.5rem;
}

.objective-item p {
    color: var(--text-secondary);
    font-size: 0.9rem;
}

.progress-section {
    margin-bottom: 4rem;
}

.progress-section h2 {
    font-size: 1.5rem;
    color: var(--text-primary);
    margin-bottom: 2rem;
}

.progress-stats {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
    gap: 1.5rem;
    margin-bottom: 2rem;
}

.stat-item {
    text-align: center;
    background: var(--card-bg);
    padding: 1.5rem;
    border-radius: var(--border-radius);
    border: 1px solid var(--border-color);
    box-shadow: var(--shadow-sm);
}

.stat-number {
    font-size: 2rem;
    font-weight: 700;
    color: var(--primary-color);
    display: block;
    margin-bottom: 0.5rem;
}

.stat-label {
    color: var(--text-secondary);
    font-weight: 500;
}

.chapters-section h2 {
    font-size: 1.5rem;
    color: var(--text-primary);
    margin-bottom: 2rem;
}

.chapter-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(350px, 1fr));
    gap: 1.5rem;
    margin-bottom: 4rem;
}

.chapter-card {
    background: var(--card-bg);
    border-radius: var(--border-radius);
    padding: 1.5rem;
    border: 1px solid var(--border-color);
    box-shadow: var(--shadow-sm);
    transition: var(--transition);
    position: relative;
    overflow: hidden;
}

.chapter-card:hover {
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
}
.chapter-card::before {
    content: "";
    position: absolute;
    inset: 0 0 auto 0;
    height: 4px;
    background: var(--gradient-primary);
}

.chapter-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 1rem;
}

.chapter-number {
    font-family: 'JetBrains Mono', monospace;
    font-size: 0.875rem;
    font-weight: 500;
    color: var(--accent-color);
    background: rgba(6, 182, 212, 0.1);
    padding: 0.25rem 0.75rem;
    border-radius: 20px;
}

.difficulty-badge {
    padding: 0.25rem 0.75rem;
    border-radius: 20px;
    font-size: 0.75rem;
    font-weight: 600;
    text-transform: uppercase;
}

.difficulty-badge.beginner {
    background: rgba(16, 185, 129, 0.1);
    color: var(--success-color);
}

.difficulty-badge.intermediate {
    background: rgba(245, 158, 11, 0.1);
    color: var(--warning-color);
}

.difficulty-badge.advanced {
    background: rgba(239, 68, 68, 0.1);
    color: var(--danger-color);
}

.chapter-card h3 {
    font-size: 1.25rem;
    color: var(--text-primary);
    margin-bottom: 0.75rem;
}

.chapter-description {
    color: var(--text-secondary);
    font-size: 0.95rem;
    line-height: 1.6;
    margin-bottom: 1rem;
}

.chapter-stats {
    display: flex;
    gap: 1rem;
    margin-bottom: 1rem;
    font-size: 0.85rem;
    color: var(--text-muted);
}

/* --- Unified Subtle Content Boxes (shared) --- */
.pd-box { background: var(--bg-secondary); border: 1px solid var(--border-color); border-left: 4px solid var(--primary-color); border-radius: 8px; padding: 1.1rem 1.25rem 0.9rem; margin: 1.25rem 0; }
.pd-box h3, .pd-box h4 { margin-top: 0; }
.pd-box.theory { border-left-color: var(--warning-color); background: linear-gradient(135deg,#fff7ed,#fffbeb); }
.pd-box.definition { border-left-color: var(--primary-color); background: linear-gradient(135deg,#f1f5f9,#eff6ff); }
.pd-box.formula { border-left-color: #7c3aed; background: linear-gradient(135deg,#f5f3ff,#ede9fe); }
.pd-box.practical { border-left-color: #059669; background: linear-gradient(135deg,#f0fdf4,#dcfce7); }
.pd-box.exercise { border-left-color: var(--danger-color); background: linear-gradient(135deg,#fef2f2,#fee2e2); }
.pd-box.units { border-left-color: #0d9488; background: linear-gradient(135deg,#f0fdfa,#ccfbf1); }
.pd-box.precision { border-left-color: #f43f5e; background: linear-gradient(135deg,#fff1f2,#ffe4e6); }
.pd-box.dimensional { border-left-color: #0284c7; background: linear-gradient(135deg,#f0f9ff,#e0f2fe); }

[data-theme="dark"] .pd-box { background: var(--bg-secondary); box-shadow: none; }
[data-theme="dark"] .pd-box.theory { background: var(--bg-secondary); }
[data-theme="dark"] .pd-box.definition { background: var(--bg-secondary); }
[data-theme="dark"] .pd-box.formula { background: var(--bg-secondary); }
[data-theme="dark"] .pd-box.practical { background: var(--bg-secondary); }
[data-theme="dark"] .pd-box.exercise { background: var(--bg-secondary); }
[data-theme="dark"] .pd-box.units { background: var(--bg-secondary); }
[data-theme="dark"] .pd-box.precision { background: var(--bg-secondary); }
[data-theme="dark"] .pd-box.dimensional { background: var(--bg-secondary); }

.chapter-topics {
    display: flex;
    flex-wrap: wrap;
    gap: 0.5rem;
    margin-bottom: 1.5rem;
}

.topic-tag {
    font-size: 0.75rem;
    background: var(--bg-tertiary);
    color: var(--text-muted);
    padding: 0.25rem 0.5rem;
    border-radius: 6px;
    font-weight: 500;
    border: 1px dashed var(--border-color);
}

.chapter-actions {
    display: flex;
    align-items: center;
    gap: 1rem;
}

.progress-indicator {
    flex: 1;
    height: 6px;
    background: var(--bg-tertiary);
    border-radius: 3px;
    overflow: hidden;
}

.progress-bar {
    height: 100%;
    background: var(--gradient-primary);
    border-radius: 3px;
    transition: width 0.3s ease;
    width: 0%;
}

.quick-access-section h2 {
    font-size: 1.5rem;
    color: var(--text-primary);
    margin-bottom: 2rem;
}

.quick-access-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
    gap: 1.5rem;
}

.quick-access-card {
    background: var(--card-bg);
    border: 1px solid var(--border-color);
    border-radius: var(--border-radius);
    padding: 1.5rem;
    text-decoration: none;
    color: var(--text-primary);
    transition: var(--transition);
    text-align: center;
    box-shadow: var(--shadow-sm);
}

.quick-access-card:hover {
    transform: translateY(-2px);
    box-shadow: var(--shadow-md);
    border-color: var(--border-hover);
}

.access-icon {
    font-size: 2rem;
    display: block;
    margin-bottom: 1rem;
}

.quick-access-card h3 {
    font-size: 1.1rem;
    margin-bottom: 0.5rem;
}

.quick-access-card p {
    color: var(--text-secondary);
    font-size: 0.9rem;
}

/* --- Utility Classes --- */
.btn {
    display: inline-flex;
    align-items: center;
    gap: 0.5rem;
    padding: 0.75rem 1.5rem;
    background: var(--primary-color);
    color: white;
    text-decoration: none;
    border-radius: var(--border-radius);
    font-weight: 500;
    transition: var(--transition);
    box-shadow: var(--shadow-sm);
    border: none;
    cursor: pointer;
}
.btn:hover { background: var(--primary-dark); transform: translateY(-2px); box-shadow: var(--shadow-md); }
.btn-secondary { background: var(--card-bg); color: var(--text-primary); border: 1px solid var(--border-color); }
.btn-secondary:hover { background: var(--bg-tertiary); border-color: var(--border-hover); }
.btn-outline { background: transparent; color: var(--text-primary); border: 1px solid var(--border-color); }
.btn-outline:hover { background: var(--bg-secondary); }
.badge { display: inline-block; padding: 0.25rem 0.5rem; border-radius: 999px; font-size: var(--font-size-xs); color: var(--text-secondary); background: var(--bg-tertiary); }
.muted { color: var(--text-muted); }
.card { background: var(--card-bg); border: 1px solid var(--border-color); border-radius: var(--border-radius); box-shadow: var(--shadow-sm); }
.shadow-hover:hover { box-shadow: var(--shadow-md); transform: translateY(-2px); transition: var(--transition); }
.rounded { border-radius: var(--border-radius); }
.max-w-prose { max-width: 75ch; }
.prose { font-size: var(--font-size-lg); line-height: 1.75; }

/* --- Generic Content Styling --- */
.main-content {
    padding: 2rem 0;
}
.section {
    margin-bottom: 3rem;
}
.math {
    font-family: 'JetBrains Mono', monospace;
    font-style: italic;
    font-weight: 500;
}
.equation {
    font-family: 'JetBrains Mono', monospace;
    font-size: 1.1em;
    font-weight: 500;
    background: var(--bg-secondary);
    padding: 0.2em 0.4em;
    border-radius: 4px;
    border: 1px solid var(--border-color);
    display: inline-block;
}

/* Animation for scroll reveal */
.fade-in {
    opacity: 0;
    transform: translateY(30px);
    transition: opacity 0.6s ease, transform 0.6s ease;
}
.fade-in.visible {
    opacity: 1;
    transform: translateY(0);
}

/* Code & Pre */
code, kbd, samp {
    font-family: 'JetBrains Mono', ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace;
    background: var(--bg-secondary);
    border: 1px solid var(--border-color);
    padding: 0.15em 0.35em;
    border-radius: 6px;
    color: var(--text-primary);
}
pre {
    background: var(--bg-secondary);
    color: var(--text-primary);
    border: 1px solid var(--border-color);
    border-radius: 10px;
    padding: 1rem;
    overflow: auto;
    box-shadow: var(--shadow-sm);
}

/* Blockquote */
blockquote {
    border-left: 4px solid var(--accent-color);
    background: var(--bg-secondary);
    padding: 1rem 1.25rem;
    margin: 1rem 0;
    border-radius: 8px;
    color: var(--text-secondary);
}

/* Tables */
table { width: 100%; border-collapse: collapse; margin: 1.25rem 0; background: var(--card-bg); border-radius: 10px; overflow: hidden; box-shadow: var(--shadow-sm); }
thead { background: var(--bg-secondary); }
th, td { border: 1px solid var(--border-color); padding: 0.75rem 0.9rem; text-align: left; }
tr:nth-child(even) td { background: var(--bg-secondary); }

/* --- Images & Media --- */
img {
    max-width: 100%;
    height: auto;
    border-radius: var(--border-radius);
}

/* --- About Page Styles --- */
.about-section {
    max-width: 800px;
    margin: 0 auto;
    text-align: center;
}

.profile-image {
    width: 150px;
    height: 150px;
    border-radius: 50%;
    object-fit: cover;
    margin-bottom: 1.5rem;
    border: 4px solid var(--primary-color);
    box-shadow: var(--shadow-lg);
}

.about-section h2 {
    font-size: 2rem;
    font-weight: 600;
    color: var(--text-primary);
    margin-bottom: 0.5rem;
}

.about-section h3 {
    font-size: 1.2rem;
    font-weight: 500;
    color: var(--primary-color);
    margin-bottom: 1.5rem;
}

.about-section p {
    color: var(--text-secondary);
    font-size: 1.1rem;
    line-height: 1.8;
    text-align: justify;
}

/* YouTube and video embeds */
.video-embed {
    position: relative;
    padding-bottom: 56.25%; /* 16:9 aspect ratio */
    height: 0;
    overflow: hidden;
    border-radius: var(--border-radius);
    margin: 2rem 0;
}

.video-embed iframe {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    border: none;
}

/* Skip to main content for screen readers */
.skip-link {
    position: absolute;
    top: -40px;
    left: 6px;
    background: var(--primary-color);
    color: white;
    padding: 8px;
    border-radius: 4px;
    text-decoration: none;
    font-weight: 600;
    z-index: 1000;
    transition: top 0.3s ease;
}

.skip-link:focus {
    top: 6px;
}
figure { margin: 1.25rem 0; }
figcaption { text-align: center; color: var(--text-muted); font-size: var(--font-size-sm); margin-top: 0.5rem; }

/* TOC utility (if used) */
.toc { position: sticky; top: 96px; align-self: start; background: var(--card-bg); border: 1px solid var(--border-color); border-radius: 10px; padding: 1rem; max-height: calc(100vh - 120px); overflow: auto; }
.toc a { color: var(--text-secondary); text-decoration: none; display: block; padding: 0.25rem 0; }
.toc a:hover { color: var(--primary-color); }

/* Focus styles, accessibility */
:focus-visible { outline: 2px solid var(--primary-color); outline-offset: 2px; }
::selection { background: rgba(37, 99, 235, 0.2); }

/* Motion preferences */
@media (prefers-reduced-motion: reduce) {
    * { transition: none !important; animation: none !important; }
}

/* Responsive Design */
@media (max-width: 768px) {
    .container { padding: 0 1rem; }
    
    /* Mobile Navigation */
    .nav-toggle { display: flex; }
    .nav-links { 
        display: none;
        position: absolute;
        top: 100%;
        left: 0;
        right: 0;
        background: var(--card-bg);
        border-top: 1px solid var(--border-color);
        flex-direction: column;
        padding: 1rem 0;
        box-shadow: var(--shadow-md);
    }
    .nav-links.active { display: flex; }
    .nav-content { flex-direction: row; justify-content: space-between; }
    
    .header { padding: 2rem 0; }
    .chapter-grid { grid-template-columns: 1fr; }
    .objectives-grid { grid-template-columns: 1fr; }
    .progress-stats { grid-template-columns: repeat(2, 1fr); }
    .quick-access-grid { grid-template-columns: 1fr; }
    .chapter-actions { flex-direction: column; align-items: stretch; }
    .progress-indicator { order: -1; margin-bottom: 1rem; }
}

@media (max-width: 480px) {
    .progress-stats { grid-template-columns: 1fr; }
    .chapter-stats { flex-direction: column; gap: 0.5rem; }
}

/* (Removed legacy theme toggle style block) */