/* ============================================================
   Python Learning Hub — Premium White Design System
   ============================================================ */

/* Google Fonts */
@import url('https://fonts.googleapis.com/css2?family=Inter:wght@300;400;500;600;700;800;900&family=Fira+Code:wght@400;500;600&display=swap');

:root {
    /* Colors */
    --white: #ffffff;
    --bg: #f8f9fc;
    --bg-alt: #f1f3f9;
    --border: #e5e9f0;
    --border-strong: #c8d0df;

    /* Text */
    --text-primary: #1a202c;
    --text-secondary: #4a5568;
    --text-muted: #718096;

    /* Accent Colors */
    --accent: #3b82f6;
    --accent-dark: #1d4ed8;
    --accent-light: #eff6ff;
    --accent-hover: #2563eb;

    --green: #10b981;
    --green-light: #ecfdf5;
    --yellow: #f59e0b;
    --yellow-light: #fffbeb;
    --purple: #8b5cf6;
    --purple-light: #f5f3ff;
    --red: #ef4444;
    --red-light: #fef2f2;

    /* Code */
    --code-bg: #0f172a;
    --code-text: #e2e8f0;
    --code-keyword: #7dd3fc;
    --code-string: #86efac;
    --code-comment: #64748b;
    --code-number: #fdba74;
    --code-function: #c4b5fd;

    /* Sidebar */
    --sidebar-width: 280px;
    --sidebar-bg: #ffffff;
    --sidebar-border: #e5e9f0;
    --nav-active-bg: #eff6ff;
    --nav-hover-bg: #f8f9fc;

    /* Shadows */
    --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.07);
    --shadow-md: 0 4px 12px rgba(0, 0, 0, 0.08);
    --shadow-lg: 0 10px 30px rgba(0, 0, 0, 0.10);

    /* Transitions */
    --transition: all 0.2s cubic-bezier(0.4, 0, 0.2, 1);
}

/* ============================================================
   RESET & BASE
   ============================================================ */
*,
*::before,
*::after {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

html {
    scroll-behavior: smooth;
    font-size: 16px;
}

body {
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, system-ui, sans-serif;
    background: var(--bg);
    color: var(--text-primary);
    line-height: 1.7;
    display: flex;
    min-height: 100vh;
    -webkit-font-smoothing: antialiased;
}

/* ============================================================
   SIDEBAR
   ============================================================ */
.sidebar {
    width: var(--sidebar-width);
    min-width: var(--sidebar-width);
    background: var(--sidebar-bg);
    border-right: 1px solid var(--sidebar-border);
    height: 100vh;
    position: fixed;
    top: 0;
    left: 0;
    overflow-y: auto;
    z-index: 100;
    display: flex;
    flex-direction: column;
    box-shadow: var(--shadow-sm);
    transition: var(--transition);
}

.sidebar::-webkit-scrollbar {
    width: 4px;
}

.sidebar::-webkit-scrollbar-thumb {
    background: var(--border-strong);
    border-radius: 2px;
}

.sidebar-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 1.25rem 1.5rem;
    border-bottom: 1px solid var(--border);
    position: sticky;
    top: 0;
    background: var(--white);
    z-index: 10;
}

.logo {
    display: flex;
    align-items: center;
    gap: 0.75rem;
    text-decoration: none;
}

.logo-icon {
    font-size: 1.75rem;
    line-height: 1;
}

.logo-text {
    display: flex;
    flex-direction: column;
}

.logo-title {
    font-size: 1.05rem;
    font-weight: 800;
    color: var(--text-primary);
    letter-spacing: -0.02em;
}

.logo-sub {
    font-size: 0.7rem;
    color: var(--text-muted);
    font-weight: 500;
    text-transform: uppercase;
    letter-spacing: 0.05em;
}

.sidebar-toggle,
.mobile-toggle {
    background: none;
    border: 1px solid var(--border);
    border-radius: 6px;
    cursor: pointer;
    color: var(--text-secondary);
    font-size: 1rem;
    padding: 0.35rem 0.5rem;
    transition: var(--transition);
}

.sidebar-toggle:hover,
.mobile-toggle:hover {
    background: var(--bg-alt);
    border-color: var(--accent);
    color: var(--accent);
}

/* Search Box */
.search-box {
    padding: 1rem 1.25rem;
    border-bottom: 1px solid var(--border);
}

.search-input {
    width: 100%;
    padding: 0.6rem 0.9rem;
    border: 1px solid var(--border-strong);
    border-radius: 8px;
    font-family: inherit;
    font-size: 0.875rem;
    color: var(--text-primary);
    background: var(--bg);
    transition: var(--transition);
    outline: none;
}

.search-input:focus {
    border-color: var(--accent);
    background: var(--white);
    box-shadow: 0 0 0 3px rgba(59, 130, 246, 0.1);
}

/* Navigation Sections */
.nav-items {
    padding: 0.75rem 0;
    flex: 1;
}

.nav-section {
    margin-bottom: 0.25rem;
}

.nav-section-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 0.6rem 1.25rem;
    font-size: 0.78rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.06em;
    color: var(--text-secondary);
    cursor: pointer;
    transition: var(--transition);
    user-select: none;
}

.nav-section-header:hover {
    color: var(--accent);
    background: var(--nav-hover-bg);
}

.chevron {
    font-size: 0.6rem;
    transition: transform 0.2s;
}

.nav-section-header.collapsed .chevron {
    transform: rotate(-90deg);
}

.nav-section-items {
    overflow: hidden;
    transition: max-height 0.3s ease;
}

.nav-section-items.hidden {
    max-height: 0 !important;
}

.nav-item {
    display: block;
    padding: 0.45rem 1.25rem 0.45rem 2rem;
    font-size: 0.875rem;
    color: var(--text-secondary);
    text-decoration: none;
    border-left: 3px solid transparent;
    transition: var(--transition);
    position: relative;
}

.nav-item:hover {
    color: var(--accent);
    background: var(--nav-hover-bg);
    border-left-color: var(--accent);
}

.nav-item.active {
    color: var(--accent);
    background: var(--nav-active-bg);
    border-left-color: var(--accent);
    font-weight: 600;
}

/* ============================================================
   MAIN CONTENT
   ============================================================ */
.main-content {
    margin-left: var(--sidebar-width);
    flex: 1;
    min-height: 100vh;
    display: flex;
    flex-direction: column;
    background: var(--white);
}

/* Topbar */
.topbar {
    display: flex;
    align-items: center;
    gap: 1rem;
    padding: 0.875rem 2rem;
    background: var(--white);
    border-bottom: 1px solid var(--border);
    position: sticky;
    top: 0;
    z-index: 50;
    box-shadow: var(--shadow-sm);
}

.mobile-toggle {
    display: none;
}

.topbar-breadcrumb {
    font-size: 0.85rem;
    color: var(--text-muted);
    font-weight: 500;
}

.topbar-progress {
    display: flex;
    align-items: center;
    gap: 0.75rem;
    margin-left: auto;
    font-size: 0.8rem;
    color: var(--text-muted);
}

.progress-text {
    font-weight: 600;
}

.progress-bar-wrap {
    width: 120px;
    height: 6px;
    background: var(--border);
    border-radius: 3px;
    overflow: hidden;
}

.progress-bar-fill {
    height: 100%;
    background: linear-gradient(to right, var(--accent), var(--green));
    border-radius: 3px;
    transition: width 0.5s ease;
}

#progressPercent {
    font-weight: 700;
    color: var(--accent);
}

/* ============================================================
   HOME HERO
   ============================================================ */
.home-hero {
    padding: 4rem 3rem;
    background: linear-gradient(135deg, var(--white) 0%, var(--accent-light) 100%);
    border-bottom: 1px solid var(--border);
    text-align: center;
}

.hero-badge {
    display: inline-block;
    background: var(--accent-light);
    color: var(--accent-dark);
    border: 1px solid rgba(59, 130, 246, 0.3);
    padding: 0.35rem 1rem;
    border-radius: 99px;
    font-size: 0.8rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    margin-bottom: 1.5rem;
}

.home-hero h1 {
    font-size: 3.25rem;
    font-weight: 900;
    color: var(--text-primary);
    letter-spacing: -0.03em;
    line-height: 1.15;
    margin-bottom: 1.25rem;
}

.hero-subtitle {
    font-size: 1.15rem;
    color: var(--text-secondary);
    max-width: 680px;
    margin: 0 auto 2rem;
    line-height: 1.8;
}

.hero-stats {
    display: flex;
    justify-content: center;
    gap: 3rem;
    margin-bottom: 2.5rem;
}

.stat-item {
    text-align: center;
}

.stat-num {
    font-size: 2rem;
    font-weight: 900;
    color: var(--accent);
    letter-spacing: -0.04em;
}

.stat-label {
    font-size: 0.8rem;
    color: var(--text-muted);
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 0.05em;
}

.hero-cta {
    display: inline-block;
    background: var(--accent);
    color: var(--white);
    padding: 0.9rem 2.25rem;
    border-radius: 10px;
    text-decoration: none;
    font-weight: 700;
    font-size: 1rem;
    transition: var(--transition);
    box-shadow: 0 4px 14px rgba(59, 130, 246, 0.3);
}

.hero-cta:hover {
    background: var(--accent-hover);
    transform: translateY(-2px);
    box-shadow: 0 8px 20px rgba(59, 130, 246, 0.4);
}

/* ============================================================
   CHAPTER GRID
   ============================================================ */
.chapters-section,
.books-section {
    padding: 3rem;
    border-bottom: 1px solid var(--border);
}

.section-title {
    font-size: 1.6rem;
    font-weight: 800;
    color: var(--text-primary);
    margin-bottom: 1.75rem;
    letter-spacing: -0.02em;
}

.chapter-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(320px, 1fr));
    gap: 1.25rem;
}

.chapter-card {
    display: flex;
    gap: 1.25rem;
    padding: 1.5rem;
    background: var(--white);
    border: 1px solid var(--border);
    border-radius: 12px;
    text-decoration: none;
    transition: var(--transition);
    box-shadow: var(--shadow-sm);
}

.chapter-card:hover {
    border-color: var(--accent);
    box-shadow: var(--shadow-lg);
    transform: translateY(-3px);
}

.chapter-icon {
    font-size: 2.25rem;
    line-height: 1;
    flex-shrink: 0;
}

.chapter-info h3 {
    font-size: 1rem;
    font-weight: 700;
    color: var(--text-primary);
    margin-bottom: 0.4rem;
}

.chapter-info p {
    font-size: 0.84rem;
    color: var(--text-secondary);
    line-height: 1.5;
    margin-bottom: 0.75rem;
}

.chapter-meta {
    font-size: 0.75rem;
    font-weight: 700;
    color: var(--accent);
    text-transform: uppercase;
    letter-spacing: 0.04em;
}

/* ============================================================
   BOOKS GRID
   ============================================================ */
.books-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(240px, 1fr));
    gap: 1.25rem;
}

.book-card {
    display: flex;
    gap: 1rem;
    padding: 1.25rem;
    background: var(--white);
    border: 1px solid var(--border);
    border-radius: 10px;
    box-shadow: var(--shadow-sm);
    transition: var(--transition);
}

.book-card:hover {
    box-shadow: var(--shadow-md);
    transform: translateY(-2px);
}

.book-spine {
    width: 8px;
    min-height: 60px;
    border-radius: 4px;
    flex-shrink: 0;
}

.book-info h4 {
    font-size: 0.9rem;
    font-weight: 700;
    color: var(--text-primary);
    margin-bottom: 0.3rem;
    line-height: 1.4;
}

.book-info p {
    font-size: 0.78rem;
    color: var(--text-secondary);
    margin-bottom: 0.6rem;
    line-height: 1.5;
}

.book-tag {
    font-size: 0.7rem;
    font-weight: 700;
    background: var(--accent-light);
    color: var(--accent-dark);
    padding: 0.2rem 0.6rem;
    border-radius: 99px;
    text-transform: uppercase;
    letter-spacing: 0.04em;
}

/* ============================================================
   CONTENT PAGE LAYOUT
   ============================================================ */
.page-content {
    padding: 2.5rem 3rem;
    max-width: 900px;
}

/* Page breadcrumb */
.breadcrumb {
    display: flex;
    align-items: center;
    gap: 0.5rem;
    font-size: 0.8rem;
    color: var(--text-muted);
    margin-bottom: 2rem;
    font-weight: 500;
}

.breadcrumb a {
    color: var(--accent);
    text-decoration: none;
}

.breadcrumb a:hover {
    text-decoration: underline;
}

.breadcrumb span {
    color: var(--border-strong);
}

/* Page Title */
.page-header {
    margin-bottom: 2.5rem;
    padding-bottom: 1.5rem;
    border-bottom: 2px solid var(--border);
}

.page-header .topic-badge {
    display: inline-block;
    font-size: 0.72rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.06em;
    padding: 0.25rem 0.75rem;
    border-radius: 99px;
    margin-bottom: 0.75rem;
    background: var(--accent-light);
    color: var(--accent-dark);
}

.page-header h1 {
    font-size: 2.5rem;
    font-weight: 900;
    color: var(--text-primary);
    letter-spacing: -0.03em;
    line-height: 1.2;
    margin-bottom: 0.75rem;
}

.page-intro {
    font-size: 1.1rem;
    color: var(--text-secondary);
    line-height: 1.8;
    max-width: 760px;
}

/* Section Headers */
.content-section {
    margin-bottom: 3.5rem;
}

.content-section h2 {
    font-size: 1.6rem;
    font-weight: 800;
    color: var(--text-primary);
    letter-spacing: -0.02em;
    margin-bottom: 1.25rem;
    padding-bottom: 0.6rem;
    border-bottom: 2px solid var(--border);
    display: flex;
    align-items: center;
    gap: 0.5rem;
}

.content-section h3 {
    font-size: 1.2rem;
    font-weight: 700;
    color: var(--text-primary);
    margin: 2rem 0 0.75rem;
}

.content-section h4 {
    font-size: 1rem;
    font-weight: 700;
    color: var(--text-secondary);
    margin: 1.5rem 0 0.5rem;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    font-size: 0.82rem;
}

.content-section p {
    color: var(--text-secondary);
    margin-bottom: 1rem;
    line-height: 1.85;
    font-size: 0.97rem;
}

.content-section ul,
.content-section ol {
    padding-left: 1.5rem;
    margin-bottom: 1.25rem;
}

.content-section li {
    color: var(--text-secondary);
    margin-bottom: 0.4rem;
    font-size: 0.95rem;
    line-height: 1.8;
}

.content-section code {
    background: var(--bg-alt);
    padding: 0.1rem 0.4rem;
    border-radius: 4px;
    font-family: 'Fira Code', monospace;
    font-size: 0.875em;
    color: var(--accent-dark);
    border: 1px solid var(--border);
}

/* ============================================================
   CODE BLOCKS
   ============================================================ */
.code-block-wrapper {
    margin: 1.75rem 0;
    border-radius: 10px;
    overflow: hidden;
    box-shadow: var(--shadow-md);
    border: 1px solid var(--border);
}

.code-block-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 0.6rem 1.25rem;
    background: #1e293b;
    border-bottom: 1px solid rgba(255, 255, 255, 0.06);
}

.code-lang-badge {
    font-size: 0.72rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.08em;
    color: var(--code-keyword);
}

.code-dots {
    display: flex;
    gap: 0.35rem;
}

.code-dots span {
    width: 10px;
    height: 10px;
    border-radius: 50%;
}

.code-dots span:nth-child(1) {
    background: #ff5f57;
}

.code-dots span:nth-child(2) {
    background: #ffbd2e;
}

.code-dots span:nth-child(3) {
    background: #28c840;
}

.copy-btn {
    background: rgba(255, 255, 255, 0.08);
    border: none;
    color: #94a3b8;
    font-size: 0.75rem;
    padding: 0.25rem 0.75rem;
    border-radius: 4px;
    cursor: pointer;
    font-family: inherit;
    transition: var(--transition);
}

.copy-btn:hover {
    background: rgba(255, 255, 255, 0.15);
    color: #fff;
}

pre.code-block {
    background: var(--code-bg);
    color: var(--code-text);
    padding: 1.5rem;
    overflow-x: auto;
    font-family: 'Fira Code', 'Courier New', monospace;
    font-size: 0.88rem;
    line-height: 1.8;
    margin: 0;
}

pre.code-block::-webkit-scrollbar {
    height: 4px;
}

pre.code-block::-webkit-scrollbar-thumb {
    background: #334155;
    border-radius: 2px;
}

/* Syntax highlighting */
.kw {
    color: #7dd3fc;
    font-weight: 600;
}

/* keyword */
.fn {
    color: #c4b5fd;
}

/* function */
.st {
    color: #86efac;
}

/* string */
.cm {
    color: #64748b;
    font-style: italic;
}

/* comment */
.nm {
    color: #fdba74;
}

/* number */
.bi {
    color: #fda4af;
}

/* built-in */
.cl {
    color: #fde68a;
}

/* class */

/* Output Block */
.output-block {
    margin: 0;
    background: #f0fdf4;
    border: 1px solid #bbf7d0;
    border-top: none;
    padding: 1.25rem 1.5rem;
    font-family: 'Fira Code', monospace;
    font-size: 0.85rem;
    color: #065f46;
    line-height: 1.7;
}

.output-label {
    font-size: 0.7rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.08em;
    color: #10b981;
    margin-bottom: 0.5rem;
    display: flex;
    align-items: center;
    gap: 0.4rem;
}

/* ============================================================
   CALLOUT BOXES
   ============================================================ */
.callout {
    padding: 1.25rem 1.5rem;
    border-radius: 10px;
    margin: 1.5rem 0;
    border: 1px solid;
    display: flex;
    gap: 1rem;
    align-items: flex-start;
}

.callout-icon {
    font-size: 1.2rem;
    flex-shrink: 0;
    margin-top: 0.1rem;
}

.callout-content {
    flex: 1;
}

.callout-content strong {
    display: block;
    font-size: 0.85rem;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    margin-bottom: 0.4rem;
}

.callout-content p,
.callout-content li {
    font-size: 0.9rem !important;
    margin: 0 !important;
    line-height: 1.7 !important;
}

.callout.note {
    background: var(--accent-light);
    border-color: #93c5fd;
    color: #1e40af;
}

.callout.tip {
    background: var(--green-light);
    border-color: #6ee7b7;
    color: #065f46;
}

.callout.warning {
    background: var(--yellow-light);
    border-color: #fcd34d;
    color: #92400e;
}

.callout.important {
    background: var(--purple-light);
    border-color: #c4b5fd;
    color: #5b21b6;
}

/* ============================================================
   TABLES
   ============================================================ */
.data-table {
    width: 100%;
    border-collapse: collapse;
    margin: 1.5rem 0;
    font-size: 0.9rem;
    box-shadow: var(--shadow-sm);
    border-radius: 8px;
    overflow: hidden;
}

.data-table thead th {
    background: var(--text-primary);
    color: var(--white);
    padding: 0.875rem 1rem;
    text-align: left;
    font-weight: 700;
    font-size: 0.82rem;
    text-transform: uppercase;
    letter-spacing: 0.05em;
}

.data-table td {
    padding: 0.75rem 1rem;
    border-bottom: 1px solid var(--border);
    color: var(--text-secondary);
}

.data-table tbody tr:last-child td {
    border-bottom: none;
}

.data-table tbody tr:hover {
    background: var(--bg-alt);
}

.data-table code {
    background: var(--bg-alt);
    padding: 0.1rem 0.4rem;
    border-radius: 4px;
    font-family: 'Fira Code', monospace;
    font-size: 0.8em;
}

/* ============================================================
   RETURN VALUE BADGE
   ============================================================ */
.return-value-box {
    background: linear-gradient(135deg, var(--purple-light) 0%, var(--accent-light) 100%);
    border: 1px solid #c4b5fd;
    border-radius: 10px;
    padding: 1.25rem 1.5rem;
    margin: 1.5rem 0;
}

.return-value-box .rv-label {
    font-size: 0.72rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.08em;
    color: var(--purple);
    margin-bottom: 0.5rem;
    display: flex;
    align-items: center;
    gap: 0.4rem;
}

.return-value-box code {
    font-family: 'Fira Code', monospace;
    font-size: 0.9rem;
    color: var(--purple);
    background: transparent;
    border: none;
}

/* ============================================================
   STEP-BY-STEP EXPLANATION
   ============================================================ */
.steps-list {
    counter-reset: step-counter;
    list-style: none;
    padding-left: 0;
    margin: 1.5rem 0;
}

.steps-list li {
    counter-increment: step-counter;
    padding: 1rem 1rem 1rem 3.5rem;
    margin-bottom: 0.75rem;
    background: var(--bg);
    border: 1px solid var(--border);
    border-radius: 10px;
    position: relative;
    font-size: 0.93rem;
    color: var(--text-secondary);
    line-height: 1.7;
}

.steps-list li::before {
    content: counter(step-counter);
    position: absolute;
    left: 0.75rem;
    top: 50%;
    transform: translateY(-50%);
    width: 1.75rem;
    height: 1.75rem;
    background: var(--accent);
    color: var(--white);
    font-size: 0.8rem;
    font-weight: 700;
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
}

/* ============================================================
   PAGE NAVIGATION (PREV / NEXT)
   ============================================================ */
.page-nav {
    display: flex;
    justify-content: space-between;
    gap: 1rem;
    padding: 2.5rem 3rem;
    border-top: 1px solid var(--border);
    background: var(--bg);
    margin-top: auto;
}

.page-nav a {
    display: flex;
    flex-direction: column;
    padding: 1rem 1.5rem;
    border: 1px solid var(--border);
    border-radius: 10px;
    text-decoration: none;
    transition: var(--transition);
    background: var(--white);
    flex: 1;
    max-width: 260px;
}

.page-nav a:hover {
    border-color: var(--accent);
    box-shadow: var(--shadow-md);
}

.page-nav .nav-direction {
    font-size: 0.72rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.06em;
    color: var(--text-muted);
    margin-bottom: 0.35rem;
}

.page-nav .nav-topic {
    font-size: 0.95rem;
    font-weight: 700;
    color: var(--accent);
}

.page-nav a.next {
    text-align: right;
    margin-left: auto;
}

/* ============================================================
   FOOTER
   ============================================================ */
.footer {
    padding: 2rem 3rem;
    border-top: 1px solid var(--border);
    text-align: center;
    font-size: 0.85rem;
    color: var(--text-muted);
    background: var(--bg);
}

/* ============================================================
   DIFFICULTY BADGE
   ============================================================ */
.difficulty-badge {
    display: inline-flex;
    align-items: center;
    gap: 0.3rem;
    padding: 0.25rem 0.75rem;
    border-radius: 99px;
    font-size: 0.72rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.05em;
}

.difficulty-badge.beginner {
    background: var(--green-light);
    color: var(--green);
}

.difficulty-badge.intermediate {
    background: var(--yellow-light);
    color: var(--yellow);
}

.difficulty-badge.advanced {
    background: var(--red-light);
    color: var(--red);
}

/* ============================================================
   TOC (Table of Contents)
   ============================================================ */
.toc-box {
    background: var(--bg-alt);
    border: 1px solid var(--border);
    border-radius: 10px;
    padding: 1.5rem;
    margin-bottom: 2.5rem;
}

.toc-box h4 {
    font-size: 0.8rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.07em;
    color: var(--text-muted);
    margin-bottom: 1rem;
}

.toc-box ol {
    margin: 0;
    padding-left: 1.25rem;
}

.toc-box li {
    font-size: 0.88rem;
    margin-bottom: 0.4rem;
}

.toc-box a {
    color: var(--accent);
    text-decoration: none;
    font-weight: 500;
}

.toc-box a:hover {
    text-decoration: underline;
}

/* ============================================================
   RESPONSIVE
   ============================================================ */
@media (max-width: 900px) {
    .sidebar {
        transform: translateX(-100%);
        z-index: 200;
    }

    .sidebar.open {
        transform: translateX(0);
        box-shadow: 0 0 0 100vw rgba(0, 0, 0, 0.4);
    }

    .main-content {
        margin-left: 0;
    }

    .mobile-toggle {
        display: block;
    }

    .home-hero {
        padding: 2.5rem 1.5rem;
    }

    .home-hero h1 {
        font-size: 2rem;
    }

    .hero-stats {
        gap: 1.5rem;
    }

    .chapters-section,
    .books-section {
        padding: 2rem 1.5rem;
    }

    .page-content {
        padding: 1.5rem;
    }

    .page-nav {
        padding: 1.5rem;
    }
}

/* ============================================================
   ANIMATIONS
   ============================================================ */
@keyframes fadeInUp {
    from {
        opacity: 0;
        transform: translateY(20px);
    }

    to {
        opacity: 1;
        transform: translateY(0);
    }
}

.page-header,
.content-section,
.chapter-card,
.book-card {
    animation: fadeInUp 0.4s ease both;
}

.chapter-card:nth-child(1) {
    animation-delay: 0.05s;
}

.chapter-card:nth-child(2) {
    animation-delay: 0.1s;
}

.chapter-card:nth-child(3) {
    animation-delay: 0.15s;
}

.chapter-card:nth-child(4) {
    animation-delay: 0.2s;
}

.chapter-card:nth-child(5) {
    animation-delay: 0.25s;
}

.chapter-card:nth-child(6) {
    animation-delay: 0.3s;
}

.chapter-card:nth-child(7) {
    animation-delay: 0.35s;
}

.chapter-card:nth-child(8) {
    animation-delay: 0.4s;
}
/* ============================================================
   ADVANCED CONTENT COMPONENTS
   ============================================================ */
.textbook-list {
    margin: 1.5rem 0;
    padding-left: 1.75rem;
    list-style: none;
}

.textbook-list li {
    position: relative;
    margin-bottom: 0.75rem;
    padding-left: 0.5rem;
}

.textbook-list li::before {
    content: '’!';
    position: absolute;
    left: -1.25rem;
    color: var(--accent);
    font-weight: 700;
}

.callout {
    margin: 2rem 0;
    padding: 1.25rem 1.5rem;
    border-radius: 10px;
    border-left: 4px solid;
    font-size: 0.95rem;
    line-height: 1.7;
    background: var(--bg-alt);
}

.callout strong {
    display: block;
    margin-bottom: 0.5rem;
    text-transform: uppercase;
    font-size: 0.75rem;
    letter-spacing: 0.05em;
}

.callout-note {
    border-color: var(--accent);
    background: var(--accent-light);
}

.callout-warning {
    border-color: var(--yellow);
    background: var(--yellow-light);
}

.callout-definition {
    border-color: var(--purple);
    background: var(--purple-light);
}

.callout-important {
    border-color: var(--red);
    background: var(--red-light);
}



/* ============================================================
   WEB-FIRST PREMIUM COMPONENTS
   ============================================================ */
.side-heading {
    display: flex;
    align-items: center;
    gap: 0.75rem;
    font-size: 1.4rem;
    font-weight: 800;
    color: var(--text-main);
    margin: 3rem 0 1.5rem 0;
    padding-bottom: 0.75rem;
    border-bottom: 2px solid var(--border);
    letter-spacing: -0.02em;
}

.content-card {
    background: #ffffff;
    border: 1px solid var(--border);
    border-radius: 12px;
    padding: 2rem;
    margin-bottom: 2rem;
    box-shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.05);
    line-height: 1.8;
    color: #374151;
}

.content-card p {
    margin-bottom: 1.25rem;
}

.content-card p:last-child {
    margin-bottom: 0;
}

.content-card strong {
    color: var(--accent);
    font-weight: 600;
}

.code-block-wrapper {
    margin: 2rem 0;
    background: #1e1e1e;
    border-radius: 12px;
    overflow: hidden;
    box-shadow: 0 10px 15px -3px rgba(0, 0, 0, 0.1);
}

.code-block {
    padding: 1.5rem;
    font-family: 'Fira Code', monospace;
    font-size: 0.9rem;
    color: #d1d5db;
    overflow-x: auto;
    white-space: pre-wrap;
    word-break: break-all;
}



/* ============================================================
   GLASSMORPHISM & UI POLISH
   ============================================================ */
.code-block-wrapper {
    position: relative;
    background: rgba(30, 30, 30, 0.7);
    backdrop-filter: blur(10px);
    -webkit-backdrop-filter: blur(10px);
    border: 1px solid rgba(255, 255, 255, 0.1);
    box-shadow: 0 8px 32px 0 rgba(31, 38, 135, 0.37);
    margin: 2rem 0;
    transition: all 0.3s ease;
}

.code-block-wrapper:hover {
    transform: translateY(-5px);
    box-shadow: 0 12px 40px 0 rgba(31, 38, 135, 0.5);
}

.copy-btn {
    position: absolute;
    top: 1rem;
    right: 1rem;
    background: rgba(255, 255, 255, 0.1);
    border: 1px solid rgba(255, 255, 255, 0.2);
    color: #fff;
    padding: 0.4rem 0.75rem;
    border-radius: 6px;
    font-size: 0.75rem;
    cursor: pointer;
    transition: all 0.2s;
    z-index: 10;
    font-family: 'Inter', sans-serif;
}

.copy-btn:hover {
    background: var(--accent);
    border-color: var(--accent);
}

.copy-btn.copied {
    background: #10b981;
    border-color: #10b981;
}

/* Enhanced List Styles */
.content-card ul {
    list-style: none;
    padding-left: 1.5rem;
    margin: 1.5rem 0;
}

.content-card ul li {
    position: relative;
    margin-bottom: 0.75rem;
    padding-left: 0.5rem;
}

.content-card ul li::before {
    content: '" ';
    position: absolute;
    left: -1.25rem;
    color: var(--accent);
    font-weight: 800;
}