:root {
    --bg-primary: #0a0a0f;
    --bg-secondary: #12121a;
    --bg-tertiary: #1a1a25;
    --bg-card: rgba(255, 255, 255, 0.03);
    --text-primary: #ffffff;
    --text-secondary: #a0a0b0;
    --text-muted: #9090a0;
    --border-color: rgba(255, 255, 255, 0.08);
    --accent-primary: #049fd9;
    --accent-secondary: #0277bd;
    --accent-gradient: linear-gradient(135deg, #049fd9 0%, #0277bd 50%, #01579b 100%);
    --success: #10b981;
    --warning: #f59e0b;
    --info: #3b82f6;
    --glow-1: rgba(4, 159, 217, 0.4);
}

[data-theme="light"] {
    --bg-primary: #f8fafc;
    --bg-secondary: #ffffff;
    --bg-tertiary: #f1f5f9;
    --bg-card: rgba(0, 0, 0, 0.02);
    --text-primary: #0f172a;
    --text-secondary: #475569;
    --text-muted: #94a3b8;
    --border-color: rgba(0, 0, 0, 0.1);
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

html {
    scroll-behavior: smooth;
}

body {
    font-family: 'Inter', -apple-system, BlinkMacSystemFont, sans-serif;
    background: var(--bg-primary);
    color: var(--text-primary);
    line-height: 1.8;
}

/* Header */
.header {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    height: 60px;
    background: rgba(10, 10, 15, 0.9);
    backdrop-filter: blur(20px);
    border-bottom: 1px solid var(--border-color);
    z-index: 1000;
}

[data-theme="light"] .header {
    background: rgba(255, 255, 255, 0.95);
}

.header-content {
    max-width: 900px;
    margin: 0 auto;
    height: 100%;
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 0 20px;
}

.back-btn {
    display: flex;
    align-items: center;
    gap: 8px;
    padding: 8px 14px;
    background: var(--bg-card);
    border: 1px solid var(--border-color);
    border-radius: 8px;
    color: var(--text-primary);
    text-decoration: none;
    font-size: 0.85rem;
    font-weight: 500;
    transition: all 0.2s ease;
}

.back-btn:hover {
    background: var(--accent-primary);
    border-color: var(--accent-primary);
    color: white;
}

.back-btn svg {
    width: 16px;
    height: 16px;
}

.theme-toggle {
    width: 40px;
    height: 40px;
    border-radius: 8px;
    background: var(--bg-card);
    border: 1px solid var(--border-color);
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: all 0.2s ease;
}

.theme-toggle:hover {
    background: var(--accent-primary);
    border-color: var(--accent-primary);
}

.theme-toggle:hover svg {
    color: white;
}

.theme-toggle svg {
    width: 18px;
    height: 18px;
    color: var(--text-primary);
}

.theme-toggle .sun-icon {
    display: none;
}

[data-theme="light"] .theme-toggle .moon-icon {
    display: none;
}

[data-theme="light"] .theme-toggle .sun-icon {
    display: block;
}

/* Main */
.main {
    max-width: 900px;
    margin: 0 auto;
    padding: 80px 20px 60px;
}

/* Image Container */
.image-container {
    margin: 24px 0;
    text-align: center;
}

.image-container img {
    max-width: 100%;
    height: auto;
    border-radius: 12px;
    border: 1px solid var(--border-color);
    box-shadow: 0 4px 20px rgba(0, 0, 0, 0.3);
}

/* Light mode: add background and adjust for dark images */
[data-theme="light"] .image-container img {
    background: #1a1a25;
    padding: 8px;
    box-shadow: 0 4px 20px rgba(0, 0, 0, 0.15);
}

.image-container .image-caption {
    margin-top: 12px;
    font-size: 0.85rem;
    color: var(--text-secondary);
    font-style: italic;
}

/* Chapter Header */
.chapter-header {
    text-align: center;
    margin-bottom: 48px;
    padding-bottom: 32px;
    border-bottom: 1px solid var(--border-color);
}

.chapter-badge {
    display: inline-block;
    padding: 8px 20px;
    background: var(--accent-gradient);
    border-radius: 24px;
    font-size: 0.85rem;
    font-weight: 700;
    color: white;
    text-transform: uppercase;
    letter-spacing: 1px;
    margin-bottom: 20px;
}

.chapter-header h1 {
    font-size: 2rem;
    font-weight: 800;
    margin-bottom: 12px;
    background: var(--accent-gradient);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    background-clip: text;
    line-height: 1.3;
}

.chapter-meta {
    color: var(--text-secondary);
    font-size: 0.9rem;
}

/* Content Sections */
.content-section {
    margin-bottom: 40px;
    padding-bottom: 32px;
    border-bottom: 1px solid var(--border-color);
}

.content-section:last-of-type {
    border-bottom: none;
}

.content-section h2 {
    display: flex;
    align-items: center;
    gap: 12px;
    font-size: 1.4rem;
    font-weight: 700;
    margin-bottom: 20px;
    color: var(--accent-primary);
}

.content-section h2 svg {
    width: 24px;
    height: 24px;
}

.content-block {
    margin-bottom: 24px;
}

.content-block h3 {
    font-size: 1.1rem;
    font-weight: 600;
    margin-bottom: 12px;
    color: var(--text-primary);
}

.content-block p {
    color: var(--text-secondary);
    margin-bottom: 16px;
}

.content-block ul,
.content-block ol {
    padding-left: 24px;
    margin-bottom: 16px;
}

.content-block li {
    color: var(--text-secondary);
    margin-bottom: 8px;
}

.content-block li strong {
    color: var(--text-primary);
}

/* Objectives List */
.objectives-list {
    list-style: none;
    padding: 0;
}

.objectives-list li {
    display: flex;
    align-items: flex-start;
    gap: 12px;
    padding: 12px 16px;
    background: var(--bg-tertiary);
    border-radius: 10px;
    margin-bottom: 10px;
    color: var(--text-secondary);
}

.objectives-list li::before {
    content: "✓";
    color: var(--success);
    font-weight: bold;
    flex-shrink: 0;
}

/* Feature Grid */
.feature-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(180px, 1fr));
    gap: 16px;
    margin-top: 20px;
}

.feature-card {
    background: var(--bg-tertiary);
    border: 1px solid var(--border-color);
    border-radius: 12px;
    padding: 20px;
    text-align: center;
    transition: all 0.2s ease;
}

.feature-card:hover {
    border-color: var(--accent-primary);
    transform: translateY(-2px);
}

.feature-icon {
    width: 48px;
    height: 48px;
    background: var(--accent-gradient);
    border-radius: 12px;
    display: flex;
    align-items: center;
    justify-content: center;
    margin: 0 auto 12px;
}

.feature-icon svg {
    width: 24px;
    height: 24px;
    color: white;
}

.feature-card h4 {
    font-size: 1rem;
    margin-bottom: 6px;
}

.feature-card p {
    font-size: 0.85rem;
    color: var(--text-muted);
    margin: 0;
}

/* Architecture Block */
.architecture-block {
    background: var(--bg-tertiary);
    border: 1px solid var(--border-color);
    border-radius: 12px;
    padding: 20px;
    margin-bottom: 16px;
}

.architecture-block h4 {
    font-size: 1rem;
    margin-bottom: 12px;
    color: var(--accent-primary);
}

.architecture-block ul {
    list-style: none;
    padding: 0;
    margin: 0;
}

.architecture-block li {
    padding: 6px 0;
    color: var(--text-secondary);
}

/* Benefits List */
.benefits-list {
    display: flex;
    flex-direction: column;
    gap: 12px;
}

.benefit-item {
    display: flex;
    align-items: flex-start;
    gap: 16px;
    padding: 16px;
    background: var(--bg-tertiary);
    border: 1px solid var(--border-color);
    border-radius: 12px;
}

.benefit-icon {
    font-size: 1.5rem;
    flex-shrink: 0;
}

.benefit-item strong {
    display: block;
    margin-bottom: 4px;
}

.benefit-item p {
    font-size: 0.9rem;
    color: var(--text-muted);
    margin: 0;
}

/* Review Questions */
.review-questions {
    display: flex;
    flex-direction: column;
    gap: 12px;
}

.question-item {
    display: flex;
    align-items: flex-start;
    gap: 16px;
    padding: 16px;
    background: var(--bg-tertiary);
    border: 1px solid var(--border-color);
    border-radius: 12px;
}

.q-number {
    width: 32px;
    height: 32px;
    background: var(--accent-gradient);
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 0.85rem;
    font-weight: 700;
    color: white;
    flex-shrink: 0;
}

.question-item p {
    color: var(--text-secondary);
    margin: 0;
    padding-top: 4px;
}

/* Code Block */
.code-block {
    background: #1e1e2e;
    border-radius: 12px;
    margin: 16px 0;
    overflow: hidden;
    border: 1px solid rgba(255, 255, 255, 0.1);
}

.code-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 10px 16px;
    background: #2d2d3a;
    border-bottom: 1px solid rgba(255, 255, 255, 0.1);
}

.code-lang {
    font-size: 0.75rem;
    color: #a0a0b0;
    font-weight: 500;
    text-transform: uppercase;
}

.code-block pre {
    margin: 0;
    padding: 16px;
    overflow-x: auto;
}

.code-block code {
    font-family: 'JetBrains Mono', Consolas, monospace;
    font-size: 0.85rem;
    color: #e0e0e0;
    line-height: 1.6;
}

/* Table */
.data-table {
    width: 100%;
    border-collapse: collapse;
    margin: 16px 0;
    font-size: 0.9rem;
}

.data-table th,
.data-table td {
    padding: 12px 16px;
    text-align: left;
    border: 1px solid var(--border-color);
}

.data-table th {
    background: var(--bg-tertiary);
    font-weight: 600;
    color: var(--accent-primary);
}

.data-table td {
    color: var(--text-secondary);
}

.data-table code {
    background: var(--bg-tertiary);
    padding: 2px 6px;
    border-radius: 4px;
    font-family: 'JetBrains Mono', monospace;
    font-size: 0.85rem;
}

/* Alert */
.alert {
    display: flex;
    gap: 12px;
    padding: 14px 18px;
    border-radius: 10px;
    margin: 16px 0;
    border-left: 4px solid;
}

.alert svg {
    width: 20px;
    height: 20px;
    flex-shrink: 0;
    margin-top: 2px;
}

.alert-content {
    flex: 1;
}

.alert-content strong {
    display: block;
    margin-bottom: 4px;
}

.alert-content p {
    margin: 0;
    font-size: 0.9rem;
}

.alert.info {
    background: rgba(59, 130, 246, 0.1);
    border-color: var(--info);
}

.alert.info svg {
    color: var(--info);
}

.alert.warning {
    background: rgba(245, 158, 11, 0.1);
    border-color: var(--warning);
}

.alert.warning svg {
    color: var(--warning);
}

.alert.success {
    background: rgba(16, 185, 129, 0.1);
    border-color: var(--success);
}

.alert.success svg {
    color: var(--success);
}

/* Chapter Navigation */
.chapter-nav {
    display: flex;
    justify-content: space-between;
    margin-top: 48px;
    padding-top: 24px;
    border-top: 1px solid var(--border-color);
}

.nav-btn {
    display: flex;
    align-items: center;
    gap: 8px;
    padding: 12px 20px;
    background: var(--bg-card);
    border: 1px solid var(--border-color);
    border-radius: 10px;
    color: var(--text-primary);
    text-decoration: none;
    font-weight: 500;
    transition: all 0.2s ease;
}

.nav-btn:hover {
    background: var(--accent-primary);
    border-color: var(--accent-primary);
    color: white;
}

.nav-btn svg {
    width: 18px;
    height: 18px;
}

.nav-btn.prev {
    padding-left: 16px;
}

.nav-btn.next {
    padding-right: 16px;
}

/* Responsive */
@media (max-width: 640px) {
    .chapter-header h1 {
        font-size: 1.5rem;
    }

    .content-section h2 {
        font-size: 1.2rem;
    }

    .feature-grid {
        grid-template-columns: 1fr;
    }

    .chapter-nav {
        flex-direction: column;
        gap: 12px;
    }

    .nav-btn {
        justify-content: center;
    }
}

/* Answer Sections */
.answer-section {
    margin-top: 12px;
    margin-left: 48px;
}

.answer-toggle {
    display: inline-flex;
    align-items: center;
    gap: 6px;
    padding: 8px 16px;
    background: var(--bg-tertiary);
    border: 1px solid var(--border-color);
    border-radius: 8px;
    color: var(--accent-primary);
    font-size: 0.85rem;
    font-weight: 500;
    cursor: pointer;
    transition: all 0.2s ease;
}

.answer-toggle:hover {
    background: var(--accent-primary);
    color: white;
    border-color: var(--accent-primary);
}

.answer-toggle svg {
    width: 16px;
    height: 16px;
    transition: transform 0.2s ease;
}

.answer-toggle.active svg {
    transform: rotate(180deg);
}

.answer-content {
    display: none;
    margin-top: 12px;
    padding: 16px 20px;
    background: var(--bg-tertiary);
    border: 1px solid var(--border-color);
    border-left: 4px solid var(--accent-primary);
    border-radius: 0 10px 10px 0;
}

.answer-content.show {
    display: block;
    animation: fadeIn 0.3s ease;
}

@keyframes fadeIn {
    from {
        opacity: 0;
        transform: translateY(-8px);
    }

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

.answer-content p {
    color: var(--text-secondary);
    margin: 0 0 12px 0;
}

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

.answer-content strong {
    color: var(--success);
}

.answer-content ul {
    margin: 8px 0;
    padding-left: 20px;
}

.answer-content li {
    color: var(--text-secondary);
    margin-bottom: 6px;
}

.answer-content code {
    background: var(--bg-primary);
    padding: 2px 6px;
    border-radius: 4px;
    font-family: 'JetBrains Mono', monospace;
    font-size: 0.85rem;
}