/* ===========================
   CSS Custom Properties (Design Tokens)
   =========================== */
:root {
    /* Light Theme */
    --bg-primary: #FAFAFA;
    --bg-secondary: #F0F0F0;
    --bg-tertiary: #E8E8E8;
    --text-primary: #111111;
    --text-secondary: #333333;
    --text-tertiary: #666666;
    --accent: #4A67FF;
    --accent-hover: #3A57EF;
    --border: #E0E0E0;
    --shadow: rgba(0, 0, 0, 0.08);

    /* Typography */
    --font-sans: 'Inter', -apple-system, BlinkMacSystemFont, sans-serif;
    --font-serif: 'Source Serif 4', Georgia, serif;
    --font-size-base: 18px;
    --line-height-base: 1.7;
    --content-width: 720px;

    /* Spacing */
    --header-height: 64px;
    --sidebar-width: 280px;
    --toc-width: 240px;

    /* Transitions */
    --transition-fast: 0.15s ease;
    --transition-normal: 0.3s ease;
}

/* Dark Theme */
[data-theme="dark"] {
    --bg-primary: #0E0E0E;
    --bg-secondary: #1A1A1A;
    --bg-tertiary: #252525;
    --text-primary: #F0F0F0;
    --text-secondary: #D0D0D0;
    --text-tertiary: #999999;
    --accent: #6C8CFF;
    --accent-hover: #5C7CFF;
    --border: #2A2A2A;
    --shadow: rgba(0, 0, 0, 0.3);
}

/* ===========================
   Reset & Base Styles
   =========================== */
*,
*::before,
*::after {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}

html {
    font-size: var(--font-size-base);
    scroll-behavior: smooth;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
}

body {
    font-family: var(--font-serif);
    background: var(--bg-primary);
    color: var(--text-primary);
    line-height: var(--line-height-base);
    transition: background-color var(--transition-normal), color var(--transition-normal);
    overflow-x: hidden;
}

body.font-sans .content {
    font-family: var(--font-sans);
}

/* ===========================
   Header Styles
   =========================== */
.header {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    height: var(--header-height);
    background: var(--bg-primary);
    border-bottom: 1px solid var(--border);
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 0 24px;
    z-index: 100;
    backdrop-filter: blur(10px);
    transition: background-color var(--transition-normal), border-color var(--transition-normal);
}

.header-title h1 {
    font-family: var(--font-sans);
    font-size: 1.25rem;
    font-weight: 600;
    color: var(--text-primary);
}

/* Header Navigation */
.header-nav {
    display: flex;
    align-items: center;
    gap: 8px;
    flex: 1;
    justify-content: center;
}

.header-title {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 2px;
}

.chapter-indicator {
    font-family: var(--font-sans);
    font-size: 0.7rem;
    color: var(--text-tertiary);
    font-weight: 500;
}

.btn-nav-header {
    display: flex;
    align-items: center;
    justify-content: center;
    width: 36px;
    height: 36px;
    background: var(--bg-secondary);
    border: 1px solid var(--border);
    border-radius: 50%;
    color: var(--text-secondary);
    cursor: pointer;
    transition: all var(--transition-fast);
}

.btn-nav-header:hover {
    background: var(--accent);
    border-color: var(--accent);
    color: white;
    transform: scale(1.05);
}

.btn-nav-header:disabled {
    opacity: 0.3;
    cursor: not-allowed;
}

.btn-nav-header:disabled:hover {
    background: var(--bg-secondary);
    border-color: var(--border);
    color: var(--text-secondary);
    transform: none;
}

.header-actions {
    display: flex;
    gap: 8px;
}

.sidebar-toggle {
    display: none;
    background: none;
    border: none;
    color: var(--text-primary);
    cursor: pointer;
    padding: 8px;
    border-radius: 8px;
    transition: background-color var(--transition-fast);
}

.sidebar-toggle:hover {
    background: var(--bg-tertiary);
}

.btn-icon {
    display: flex;
    align-items: center;
    justify-content: center;
    background: none;
    border: none;
    color: var(--text-secondary);
    cursor: pointer;
    padding: 10px;
    border-radius: 50%;
    transition: all var(--transition-fast);
}

.btn-icon:hover {
    background: var(--bg-tertiary);
    color: var(--text-primary);
}

/* Theme Toggle Icons */
[data-theme="light"] .moon-icon,
[data-theme="dark"] .sun-icon {
    display: none;
}

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

/* ===========================
   Settings Panel
   =========================== */
.settings-panel {
    position: fixed;
    top: calc(var(--header-height) + 8px);
    right: 24px;
    width: 280px;
    background: var(--bg-secondary);
    border: 1px solid var(--border);
    border-radius: 12px;
    padding: 20px;
    z-index: 150;
    box-shadow: 0 8px 32px var(--shadow);
    opacity: 0;
    visibility: hidden;
    transform: translateY(-10px);
    transition: all var(--transition-normal);
}

.settings-panel.active {
    opacity: 1;
    visibility: visible;
    transform: translateY(0);
}

.settings-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 20px;
}

.settings-header h3 {
    font-family: var(--font-sans);
    font-size: 0.95rem;
    font-weight: 600;
}

.btn-close {
    background: none;
    border: none;
    font-size: 1.5rem;
    color: var(--text-tertiary);
    cursor: pointer;
    padding: 4px 8px;
    line-height: 1;
    border-radius: 4px;
    transition: color var(--transition-fast);
}

.btn-close:hover {
    color: var(--text-primary);
}

.settings-group {
    margin-bottom: 16px;
}

.settings-group label {
    display: block;
    font-family: var(--font-sans);
    font-size: 0.85rem;
    color: var(--text-tertiary);
    margin-bottom: 8px;
}

.settings-control {
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 12px;
}

.settings-control span {
    font-family: var(--font-sans);
    font-size: 0.9rem;
    font-weight: 500;
    min-width: 50px;
    text-align: center;
}

.btn-setting {
    font-family: var(--font-sans);
    font-size: 0.9rem;
    font-weight: 600;
    background: var(--bg-tertiary);
    border: none;
    color: var(--text-primary);
    padding: 8px 16px;
    border-radius: 6px;
    cursor: pointer;
    transition: all var(--transition-fast);
}

.btn-setting:hover {
    background: var(--accent);
    color: white;
}

.font-selector {
    display: flex;
    gap: 8px;
}

.btn-font {
    flex: 1;
    font-family: var(--font-sans);
    font-size: 0.85rem;
    font-weight: 500;
    background: var(--bg-tertiary);
    border: 2px solid transparent;
    color: var(--text-secondary);
    padding: 8px 12px;
    border-radius: 6px;
    cursor: pointer;
    transition: all var(--transition-fast);
}

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

.btn-font.active {
    background: var(--accent);
    color: white;
    border-color: var(--accent);
}

.btn-theme {
    flex: 1;
    font-family: var(--font-sans);
    font-size: 0.85rem;
    font-weight: 500;
    background: var(--bg-tertiary);
    border: 2px solid transparent;
    color: var(--text-secondary);
    padding: 8px 12px;
    border-radius: 6px;
    cursor: pointer;
    transition: all var(--transition-fast);
}

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

.btn-theme.active {
    background: var(--accent);
    color: white;
    border-color: var(--accent);
}

/* Fun Theme Toggle */
.theme-toggle-wrapper {
    display: flex;
    gap: 8px;
}

.btn-theme-fun {
    flex: 1;
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 4px;
    padding: 12px 16px;
    background: var(--bg-tertiary);
    border: 2px solid transparent;
    border-radius: 12px;
    cursor: pointer;
    transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
}

.btn-theme-fun:hover {
    transform: translateY(-2px);
    box-shadow: 0 4px 12px var(--shadow);
}

.btn-theme-fun .theme-icon {
    font-size: 1.5rem;
    transition: transform 0.3s ease;
}

.btn-theme-fun:hover .theme-icon {
    transform: scale(1.2);
}

.btn-theme-fun .theme-label {
    font-family: var(--font-sans);
    font-size: 0.8rem;
    font-weight: 500;
    color: var(--text-tertiary);
}

.btn-theme-fun.active {
    background: linear-gradient(135deg, var(--accent) 0%, #8b5cf6 100%);
    border-color: transparent;
    box-shadow: 0 4px 16px rgba(108, 140, 255, 0.4);
}

.btn-theme-fun.active .theme-label {
    color: white;
}

.btn-theme-fun.active .theme-icon {
    animation: bounce 0.5s ease;
}

@keyframes bounce {

    0%,
    100% {
        transform: scale(1);
    }

    50% {
        transform: scale(1.3);
    }
}

/* ===========================
   Sidebar Styles
   =========================== */
.sidebar {
    position: fixed;
    top: var(--header-height);
    left: 0;
    width: var(--sidebar-width);
    height: calc(100vh - var(--header-height));
    background: var(--bg-secondary);
    border-right: 1px solid var(--border);
    overflow-y: auto;
    z-index: 80;
    transition: transform var(--transition-normal), background-color var(--transition-normal);
}

.sidebar-header {
    padding: 24px 24px 20px;
    border-bottom: 1px solid var(--border);
    position: sticky;
    top: 0;
    background: var(--bg-secondary);
}

.sidebar-header h2 {
    font-family: var(--font-sans);
    font-size: 0.75rem;
    font-weight: 500;
    text-transform: uppercase;
    letter-spacing: 0.12em;
    color: var(--text-tertiary);
}

.sidebar-nav {
    padding: 8px 0;
}

.chapter-item {
    display: flex;
    align-items: flex-start;
    gap: 12px;
    width: 100%;
    font-family: var(--font-serif);
    font-size: 0.9rem;
    line-height: 1.45;
    color: var(--text-secondary);
    text-align: left;
    padding: 12px 20px 12px 24px;
    background: none;
    border: none;
    cursor: pointer;
    transition: all var(--transition-fast);
    border-left: 2px solid transparent;
}

.chapter-item:hover {
    background: var(--bg-tertiary);
    color: var(--text-primary);
}

.chapter-item.active {
    background: linear-gradient(90deg, var(--bg-tertiary) 0%, transparent 100%);
    color: var(--accent);
    border-left-color: var(--accent);
}

.chapter-number {
    flex-shrink: 0;
    width: 20px;
    font-family: var(--font-sans);
    font-size: 0.8rem;
    font-weight: 500;
    color: var(--text-tertiary);
    text-align: right;
}

/* TOC items when displayed in sidebar (mobile) */
.sidebar .toc-item {
    font-family: var(--font-serif);
    font-size: 0.9rem;
    line-height: 1.45;
    color: var(--text-secondary);
    text-decoration: none;
    padding: 10px 20px 10px 24px;
    border-left: 2px solid transparent;
    display: block;
    transition: all var(--transition-fast);
}

.sidebar .toc-item:hover {
    background: var(--bg-tertiary);
    color: var(--text-primary);
}

.sidebar .toc-item.active {
    color: var(--accent);
    border-left-color: var(--accent);
    background: linear-gradient(90deg, var(--bg-tertiary) 0%, transparent 100%);
}

.sidebar-overlay {
    display: none;
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: rgba(0, 0, 0, 0.5);
    z-index: 75;
    opacity: 0;
    transition: opacity var(--transition-normal);
}

/* ===========================
   Main Content Styles
   =========================== */
.main-content {
    margin-left: var(--sidebar-width);
    margin-right: var(--toc-width);
    padding-top: var(--header-height);
    min-height: 100vh;
}

.reading-progress {
    position: fixed;
    top: var(--header-height);
    left: var(--sidebar-width);
    right: var(--toc-width);
    height: 3px;
    background: var(--accent);
    width: 0%;
    z-index: 90;
    transition: width 0.1s linear;
}

.content {
    max-width: var(--content-width);
    margin: 0 auto;
    padding: 48px 32px 120px;
}

/* Typography */
.content h1 {
    font-family: var(--font-sans);
    font-size: 2.25rem;
    font-weight: 700;
    margin: 56px 0 24px;
    color: var(--text-primary);
    line-height: 1.3;
}

.content h1:first-child {
    margin-top: 0;
}

.content h2 {
    font-family: var(--font-sans);
    font-size: 1.5rem;
    font-weight: 600;
    margin: 48px 0 16px;
    color: var(--text-primary);
    line-height: 1.35;
}

.content h3 {
    font-family: var(--font-sans);
    font-size: 1.2rem;
    font-weight: 600;
    margin: 32px 0 12px;
    color: var(--text-secondary);
    line-height: 1.4;
}

.content h4 {
    font-family: var(--font-sans);
    font-size: 1rem;
    font-weight: 600;
    margin: 24px 0 8px;
    color: var(--text-secondary);
}

.content p {
    margin: 0 0 1.5em;
}

/* Content Tables */
.content table {
    width: 100%;
    border-collapse: collapse;
    margin: 24px 0;
    font-family: var(--font-sans);
    font-size: 0.9rem;
}

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

.content th {
    background: var(--bg-secondary);
    font-weight: 600;
    color: var(--text-primary);
    font-size: 0.85rem;
}

.content tr:hover td {
    background: var(--bg-secondary);
}

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

.content ul,
.content ol {
    margin: 0 0 1.5em;
    padding-left: 1.5em;
}

.content li {
    margin-bottom: 0.5em;
}

.content blockquote {
    margin: 24px 0;
    padding: 16px 24px;
    background: var(--bg-secondary);
    border-left: 4px solid var(--accent);
    border-radius: 0 8px 8px 0;
    font-style: italic;
    color: var(--text-secondary);
}

.content hr {
    border: none;
    height: 1px;
    background: var(--border);
    margin: 48px 0;
}

.content strong {
    font-weight: 600;
    color: var(--text-primary);
}

.content em {
    font-style: italic;
}

/* Content Blocks */
.example-block {
    margin: 24px 0;
    padding: 20px 24px;
    background: linear-gradient(135deg, var(--bg-secondary) 0%, var(--bg-tertiary) 100%);
    border-radius: 12px;
    border: 1px solid var(--border);
}

.example-block h4 {
    font-size: 0.85rem;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    color: var(--accent);
    margin: 0 0 12px;
}

.exercise-block {
    margin: 24px 0;
    padding: 20px 24px;
    background: var(--bg-tertiary);
    border-radius: 12px;
    border-left: 4px solid var(--accent);
}

.exercise-block h4 {
    font-size: 0.9rem;
    font-weight: 600;
    color: var(--accent);
    margin: 0 0 8px;
}

/* Chapter Navigation */
.chapter-nav {
    display: flex;
    justify-content: center;
    gap: 12px;
    max-width: var(--content-width);
    margin: 48px auto 0;
    padding: 0 20px;
}

.btn-chapter {
    flex: 1;
    max-width: 200px;
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 6px;
    font-family: var(--font-sans);
    font-size: 0.85rem;
    font-weight: 500;
    background: var(--bg-secondary);
    border: 1px solid var(--border);
    color: var(--text-secondary);
    padding: 10px 16px;
    border-radius: 8px;
    cursor: pointer;
    transition: all var(--transition-fast);
    white-space: nowrap;
}

.btn-chapter svg {
    flex-shrink: 0;
}

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

.btn-chapter:disabled {
    opacity: 0.3;
    cursor: not-allowed;
}

.btn-chapter:disabled:hover {
    background: var(--bg-secondary);
    border-color: var(--border);
    color: var(--text-secondary);
}

/* ===========================
   Table of Contents (TOC) Styles
   =========================== */
.toc {
    position: fixed;
    top: var(--header-height);
    right: 0;
    width: var(--toc-width);
    height: calc(100vh - var(--header-height));
    padding: 24px 20px;
    background: var(--bg-primary);
    border-left: 1px solid var(--border);
    overflow-y: auto;
    z-index: 70;
}

.toc h3 {
    font-family: var(--font-sans);
    font-size: 0.7rem;
    font-weight: 500;
    text-transform: uppercase;
    letter-spacing: 0.12em;
    color: var(--text-tertiary);
    margin-bottom: 16px;
}

.toc-nav {
    display: flex;
    flex-direction: column;
    gap: 2px;
}

.toc-item {
    font-family: var(--font-serif);
    font-size: 0.85rem;
    line-height: 1.4;
    color: var(--text-tertiary);
    text-decoration: none;
    padding: 6px 0 6px 12px;
    border-left: 1px solid var(--border);
    transition: all var(--transition-fast);
    display: block;
}

.toc-item:hover {
    color: var(--text-primary);
    border-left-color: var(--text-tertiary);
}

.toc-item.active {
    color: var(--accent);
    border-left-color: var(--accent);
}

.toc-item.level-2 {
    padding-left: 12px;
}

.toc-item.level-3 {
    padding-left: 24px;
    font-size: 0.8rem;
    color: var(--text-tertiary);
    opacity: 0.85;
}

/* ===========================
   Mobile Navigation
   =========================== */
.mobile-nav {
    display: none;
    position: fixed;
    bottom: 0;
    left: 0;
    right: 0;
    height: 64px;
    background: var(--bg-secondary);
    border-top: 1px solid var(--border);
    z-index: 100;
    justify-content: space-around;
    align-items: center;
    padding: 0 20px;
}

.mobile-nav-btn {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: 4px;
    background: none;
    border: none;
    color: var(--text-tertiary);
    font-family: var(--font-sans);
    font-size: 0.7rem;
    cursor: pointer;
    padding: 8px 12px;
    border-radius: 8px;
    transition: all var(--transition-fast);
}

.mobile-nav-btn:hover,
.mobile-nav-btn.active {
    color: var(--accent);
    background: var(--bg-tertiary);
}

/* ===========================
   Responsive Styles
   =========================== */
@media (max-width: 1200px) {
    .toc {
        display: none;
    }

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

    .reading-progress {
        right: 0;
    }
}

@media (max-width: 900px) {
    .sidebar {
        transform: translateX(-100%);
    }

    .sidebar.active {
        transform: translateX(0);
    }

    .sidebar-overlay.active {
        display: block;
        opacity: 1;
    }

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

    .reading-progress {
        left: 0;
    }

    .sidebar-toggle {
        display: flex;
    }

    .mobile-nav {
        display: flex;
    }

    .content {
        padding-bottom: 100px;
    }

    /* Hide desktop-only elements on mobile */
    .desktop-only {
        display: none !important;
    }
}

@media (max-width: 600px) {
    :root {
        --header-height: 56px;
        --font-size-base: 16px;
    }

    .header {
        padding: 0 12px;
    }

    /* Compact header navigation on mobile */
    .header-nav {
        gap: 4px;
    }

    .header-title h1 {
        font-size: 0.9rem !important;
        font-family: var(--font-sans) !important;
    }

    .chapter-indicator {
        font-size: 0.6rem;
    }

    .btn-nav-header {
        width: 32px;
        height: 32px;
    }

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

    /* Compact chapter navigation on mobile */
    .chapter-nav {
        gap: 8px;
        padding: 0 16px;
        margin-top: 32px;
    }

    .btn-chapter {
        padding: 8px 12px;
        font-size: 0.8rem;
        max-width: none;
    }

    .btn-chapter span {
        display: none;
    }

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

    .content {
        padding: 24px 16px 90px;
        overflow-x: hidden;
    }

    .content h1 {
        font-size: 1.5rem;
        margin: 32px 0 16px;
    }

    .content h2 {
        font-size: 1.2rem;
        margin: 28px 0 12px;
    }

    .content h3 {
        font-size: 1rem;
    }

    .chapter-nav {
        gap: 12px;
        padding: 0 16px;
        margin-top: 24px;
        margin-bottom: 20px;
    }

    .btn-chapter {
        justify-content: center;
        flex: 1;
    }

    .settings-panel {
        right: 12px;
        left: 12px;
        width: auto;
    }
}

/* ===========================
   Scrollbar Styles
   =========================== */
::-webkit-scrollbar {
    width: 8px;
    height: 8px;
}

::-webkit-scrollbar-track {
    background: var(--bg-secondary);
}

::-webkit-scrollbar-thumb {
    background: var(--border);
    border-radius: 4px;
}

::-webkit-scrollbar-thumb:hover {
    background: var(--text-tertiary);
}

/* ===========================
   Animations
   =========================== */
@keyframes fadeIn {
    from {
        opacity: 0;
        transform: translateY(10px);
    }

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

.content {
    animation: fadeIn 0.4s ease;
}

/* ===========================
   Print Styles
   =========================== */
@media print {

    .header,
    .sidebar,
    .toc,
    .mobile-nav,
    .chapter-nav,
    .reading-progress {
        display: none !important;
    }

    .main-content {
        margin: 0 !important;
    }

    .content {
        max-width: 100%;
        padding: 0;
    }
}

/* ===========================
   Loading Indicator
   =========================== */
.loading-indicator {
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    min-height: 300px;
    gap: 20px;
    color: var(--text-tertiary);
    font-family: var(--font-sans);
}

.loading-spinner {
    width: 40px;
    height: 40px;
    border: 3px solid var(--border);
    border-top-color: var(--accent);
    border-radius: 50%;
    animation: spin 1s linear infinite;
}

@keyframes spin {
    to {
        transform: rotate(360deg);
    }
}

/* ===========================
   Error Message
   =========================== */
.error-message {
    display: flex;
    align-items: center;
    justify-content: center;
    min-height: 300px;
    color: #e74c3c;
    font-family: var(--font-sans);
    text-align: center;
    padding: 40px;
}