:root {
    /* Modern Theme - Light Mode (Default) */
    --bg-primary: #f8fafc;
    --bg-secondary: #ffffff;
    --bg-tertiary: #f1f5f9;
    --text-primary: #0f172a;
    --text-secondary: #475569;
    --text-muted: #64748b;
    --accent: #6366f1; /* Indigo */
    --accent-glow: rgba(99, 102, 241, 0.15);
    --border-color: #e2e8f0;
    --card-shadow: 0 4px 20px -2px rgba(0, 0, 0, 0.05), 0 2px 8px -1px rgba(0, 0, 0, 0.03);
    --header-blur: saturate(180%) blur(12px);
    --header-bg: rgba(248, 250, 252, 0.8);
    
    /* Font stacks */
    --font-heading: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
    --font-body: Georgia, "Times New Roman", serif;
    
    /* Transitions */
    --transition-speed: 0.3s;
    --transition-ease: cubic-bezier(0.4, 0, 0.2, 1);
    
    /* Font size scaling */
    --font-scale: 1;
}

/* Modern Theme - Dark Mode */
@media (prefers-color-scheme: dark) {
    :root:not(.light-theme) {
        --bg-primary: #0b0f19;
        --bg-secondary: #111827;
        --bg-tertiary: #1f2937;
        --text-primary: #f9fafb;
        --text-secondary: #d1d5db;
        --text-muted: #9ca3af;
        --accent: #818cf8; /* Light Indigo */
        --accent-glow: rgba(129, 140, 248, 0.25);
        --border-color: #1f2937;
        --card-shadow: 0 10px 30px -10px rgba(0, 0, 0, 0.7), 0 1px 3px rgba(0, 0, 0, 0.05);
        --header-bg: rgba(11, 15, 25, 0.8);
    }
}

/* Explicit Light Theme Overrides */
:root.light-theme {
    --bg-primary: #f8fafc;
    --bg-secondary: #ffffff;
    --bg-tertiary: #f1f5f9;
    --text-primary: #0f172a;
    --text-secondary: #475569;
    --text-muted: #64748b;
    --accent: #6366f1;
    --accent-glow: rgba(99, 102, 241, 0.15);
    --border-color: #e2e8f0;
    --card-shadow: 0 4px 20px -2px rgba(0, 0, 0, 0.05), 0 2px 8px -1px rgba(0, 0, 0, 0.03);
    --header-bg: rgba(248, 250, 252, 0.8);
}

/* Explicit Dark Theme Overrides */
:root.dark-theme {
    --bg-primary: #0b0f19;
    --bg-secondary: #111827;
    --bg-tertiary: #1f2937;
    --text-primary: #f9fafb;
    --text-secondary: #d1d5db;
    --text-muted: #9ca3af;
    --accent: #818cf8;
    --accent-glow: rgba(129, 140, 248, 0.25);
    --border-color: #1f2937;
    --card-shadow: 0 10px 30px -10px rgba(0, 0, 0, 0.7), 0 1px 3px rgba(0, 0, 0, 0.05);
    --header-bg: rgba(11, 15, 25, 0.8);
}

/* ==========================================================================
   Kindle / E-Ink Mode Overrides
   Designed for maximum readability, high contrast, zero animations, and simple rendering
   ========================================================================== */
body.kindle-mode {
    --bg-primary: #ffffff !important;
    --bg-secondary: #ffffff !important;
    --bg-tertiary: #ffffff !important;
    --text-primary: #000000 !important;
    --text-secondary: #000000 !important;
    --text-muted: #333333 !important;
    --accent: #000000 !important;
    --accent-glow: transparent !important;
    --border-color: #000000 !important;
    --card-shadow: none !important;
    --header-blur: none !important;
    --header-bg: #ffffff !important;
    
    --font-heading: Georgia, serif !important;
    --font-body: Georgia, serif !important;
    
    --transition-speed: 0s !important;
}

body.kindle-mode * {
    transition: none !important;
    animation: none !important;
    text-shadow: none !important;
    box-shadow: none !important;
}

body.kindle-mode a {
    text-decoration: underline !important;
    color: #000000 !important;
}

body.kindle-mode a:hover {
    background-color: #000000 !important;
    color: #ffffff !important;
}

body.kindle-mode .card,
body.kindle-mode .chapter-card {
    border: 2px solid #000000 !important;
    border-radius: 0px !important;
    background: #ffffff !important;
    margin-bottom: 1.5rem !important;
}

body.kindle-mode button, 
body.kindle-mode .btn,
body.kindle-mode .btn-icon {
    border: 2px solid #000000 !important;
    border-radius: 0px !important;
    background: #ffffff !important;
    color: #000000 !important;
    font-weight: bold !important;
    min-height: 48px !important; /* Larger hit target for e-ink touch screens */
    padding: 12px 20px !important;
}

body.kindle-mode button:hover,
body.kindle-mode .btn:hover,
body.kindle-mode .btn-icon:hover {
    background: #000000 !important;
    color: #ffffff !important;
}

body.kindle-mode pre,
body.kindle-mode code {
    background: #ffffff !important;
    border: 1px solid #000000 !important;
    border-radius: 0 !important;
    color: #000000 !important;
}

/* ==========================================================================
   Base Layout & Styling
   ========================================================================== */

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

body {
    background-color: var(--bg-primary);
    color: var(--text-primary);
    font-family: var(--font-body);
    font-size: calc(18px * var(--font-scale, 1));
    line-height: 1.6;
    transition: background-color var(--transition-speed) var(--transition-ease),
                color var(--transition-speed) var(--transition-ease);
}

h1, h2, h3, h4, h5, h6 {
    font-family: var(--font-heading);
    font-weight: 700;
    line-height: 1.25;
    color: var(--text-primary);
    margin-top: 1.5rem;
    margin-bottom: 0.75rem;
}

h1 { font-size: 2.2rem; }
h2 { font-size: 1.6rem; border-bottom: 1px solid var(--border-color); padding-bottom: 0.5rem; }
h3 { font-size: 1.3rem; }

p {
    margin-bottom: 1.25rem;
}

a {
    color: var(--accent);
    text-decoration: none;
    transition: color var(--transition-speed) var(--transition-ease);
}

a:hover {
    text-decoration: underline;
}

a:focus-visible,
button:focus-visible {
    outline: 3px solid var(--accent);
    outline-offset: 3px;
}

body.kindle-mode a:focus-visible,
body.kindle-mode button:focus-visible {
    outline-color: #000000;
}

/* ==========================================================================
   Header and Navigation
   ========================================================================== */

header {
    position: sticky;
    top: 0;
    z-index: 100;
    background-color: var(--header-bg);
    backdrop-filter: var(--header-blur);
    -webkit-backdrop-filter: var(--header-blur);
    border-bottom: 1px solid var(--border-color);
    padding: 0.75rem 1.5rem;
    transition: background-color var(--transition-speed) var(--transition-ease),
                border-color var(--transition-speed) var(--transition-ease);
}

.header-container {
    max-width: 900px;
    margin: 0 auto;
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.logo-section {
    display: flex;
    align-items: center;
    gap: 0.75rem;
}

.logo-link {
    font-family: var(--font-heading);
    font-weight: 800;
    font-size: 1.3rem;
    color: var(--text-primary);
    display: flex;
    align-items: center;
}

.logo-link:hover {
    text-decoration: none;
}

.logo-badge {
    background-color: var(--accent);
    color: #ffffff;
    padding: 2px 8px;
    border-radius: 4px;
    font-size: 0.75rem;
    font-weight: 700;
    margin-left: 0.5rem;
}

body.kindle-mode .logo-badge {
    border: 1px solid #000000;
    background: #ffffff;
    color: #000000;
}

.controls {
    display: flex;
    align-items: center;
    flex-wrap: wrap;
    justify-content: flex-end;
    gap: 0.5rem;
}

.btn-icon {
    background: transparent;
    border: 1px solid var(--border-color);
    color: var(--text-secondary);
    padding: 0.5rem 0.75rem;
    border-radius: 8px;
    cursor: pointer;
    font-family: var(--font-heading);
    font-size: 0.9rem;
    display: inline-flex;
    align-items: center;
    gap: 0.5rem;
    min-height: 44px;
    transition: all var(--transition-speed) var(--transition-ease);
}

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

.genz-link.active {
    background-color: var(--accent);
    border-color: var(--accent);
    color: #ffffff;
    font-weight: 700;
}

body.kindle-mode .genz-link.active {
    background: #000000 !important;
    color: #ffffff !important;
}

/* ==========================================================================
   Main Content Layout
   ========================================================================== */

main {
    max-width: 760px;
    margin: 2rem auto;
    padding: 0 1.5rem;
}

.hero {
    text-align: center;
    margin-bottom: 3rem;
    padding: 3rem 1.5rem;
    background: radial-gradient(circle at top right, var(--accent-glow), transparent 60%);
    border-radius: 16px;
    border: 1px solid var(--border-color);
}

body.kindle-mode .hero {
    background: transparent !important;
    border: 2px solid #000000 !important;
    border-radius: 0px !important;
    padding: 2rem 1rem !important;
}

.hero h1 {
    font-size: 2.8rem;
    letter-spacing: -0.03em;
    margin-bottom: 1rem;
}

.hero p {
    font-size: 1.2rem;
    color: var(--text-secondary);
    max-width: 600px;
    margin: 0 auto;
}

/* Table of Contents / Chapter Grid */
.chapter-grid {
    display: flex;
    flex-direction: column;
    gap: 1.5rem;
    margin-top: 2rem;
}

.chapter-card {
    position: relative;
    cursor: pointer;
    background-color: var(--bg-secondary);
    border: 1px solid var(--border-color);
    border-radius: 12px;
    padding: 1.5rem;
    display: flex;
    gap: 1.5rem;
    box-shadow: var(--card-shadow);
    transition: transform var(--transition-speed) var(--transition-ease),
                border-color var(--transition-speed) var(--transition-ease);
}

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

.chapter-card:hover .chapter-title a {
    color: var(--accent);
}

.chapter-number {
    font-family: var(--font-heading);
    font-size: 2rem;
    font-weight: 800;
    color: var(--text-muted);
    min-width: 40px;
    text-align: center;
    line-height: 1;
}

.chapter-info {
    flex-grow: 1;
}

.chapter-title {
    margin-top: 0;
    margin-bottom: 0.5rem;
}

.chapter-title a {
    color: var(--text-primary);
}

.chapter-title a:hover {
    color: var(--accent);
    text-decoration: none;
}

.chapter-description {
    color: var(--text-secondary);
    font-size: 1rem;
    margin-bottom: 0;
}

/* Doom bonus card accent */
.doom-card {
    border-left: 4px solid var(--accent);
    background: linear-gradient(135deg, var(--bg-secondary), var(--accent-glow));
}

body.kindle-mode .doom-card {
    border-left: 6px solid #000000 !important;
    background: #ffffff !important;
}

/* Reading Layout */
.chapter-header {
    margin-bottom: 2.5rem;
}

.chapter-meta {
    font-family: var(--font-heading);
    font-size: 0.95rem;
    color: var(--text-muted);
    margin-bottom: 1rem;
    display: flex;
    gap: 1rem;
}

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

.chapter-body blockquote {
    border-left: 4px solid var(--accent);
    padding-left: 1.25rem;
    margin: 1.5rem 0;
    font-style: italic;
    color: var(--text-secondary);
}

body.kindle-mode .chapter-body blockquote {
    border-left: 4px solid #000000 !important;
    color: #000000 !important;
}

.chapter-body ul,
.chapter-body ol {
    margin-bottom: 1.5rem;
    padding-left: 2rem;
}

.chapter-body li {
    margin-bottom: 0.5rem;
}

.chapter-body img {
    max-width: 100%;
    height: auto;
    border-radius: 8px;
    margin: 2rem 0;
    border: 1px solid var(--border-color);
}

body.kindle-mode .chapter-body img {
    border: 2px solid #000000 !important;
    border-radius: 0px !important;
    filter: grayscale(100%) contrast(120%) !important;
}

.code-container {
    margin: 1.5rem 0;
}

pre {
    background-color: var(--bg-tertiary);
    border: 1px solid var(--border-color);
    border-radius: 8px;
    padding: 1.25rem;
    overflow-x: auto;
    font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, monospace;
    font-size: 0.9rem;
    line-height: 1.5;
}

code {
    font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, monospace;
    font-size: 0.9em;
    background-color: var(--bg-tertiary);
    padding: 0.2em 0.4em;
    border-radius: 4px;
}

/* Key Concepts Box */
.concepts-box {
    background-color: var(--bg-tertiary);
    border-left: 4px solid var(--accent);
    border-radius: 0 8px 8px 0;
    padding: 1.5rem;
    margin: 2rem 0;
}

body.kindle-mode .concepts-box {
    border: 2px solid #000000 !important;
    border-left: 8px solid #000000 !important;
    border-radius: 0px !important;
    background: #ffffff !important;
}

.concepts-box h4 {
    margin-top: 0;
    text-transform: uppercase;
    font-size: 0.85rem;
    letter-spacing: 0.05em;
    color: var(--accent);
    margin-bottom: 0.75rem;
}

body.kindle-mode .concepts-box h4 {
    color: #000000 !important;
}

.concepts-box ul {
    margin-bottom: 0;
    list-style-type: square;
}

.sources-list {
    margin-top: 2.5rem;
    padding-top: 1.5rem;
    border-top: 1px solid var(--border-color);
}

.sources-list h2 {
    border-bottom: 0;
    padding-bottom: 0;
}

.sources-list ul {
    font-size: 0.95rem;
}

body.kindle-mode .sources-list {
    border-top: 2px solid #000000 !important;
}

/* Navigation Footer at bottom of chapters */
.chapter-navigation {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-top: 4rem;
    padding-top: 2rem;
    border-top: 1px solid var(--border-color);
}

.nav-link-card {
    position: relative;
    cursor: pointer;
    display: flex;
    flex-direction: column;
    max-width: 45%;
}

.nav-link-card.next {
    text-align: right;
    align-items: flex-end;
}

.nav-label {
    font-family: var(--font-heading);
    font-size: 0.8rem;
    color: var(--text-muted);
    text-transform: uppercase;
    letter-spacing: 0.05em;
}

.nav-title {
    font-weight: 700;
    font-size: 1.1rem;
    margin-top: 0.25rem;
    color: var(--text-primary);
    transition: color var(--transition-speed) var(--transition-ease);
}

.nav-link-card:hover .nav-title {
    color: var(--accent);
}

/* Stretched link to make the entire card / block clickable */
.chapter-card .chapter-title a::after,
.nav-link-card a.nav-title::after {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    z-index: 1;
}

/* Footer Section */
footer {
    text-align: center;
    padding: 3rem 1.5rem;
    margin-top: 6rem;
    border-top: 1px solid var(--border-color);
    color: var(--text-muted);
    font-size: 0.9rem;
    font-family: var(--font-heading);
}

footer p {
    margin-bottom: 0.5rem;
}

/* Quick Control Panel for Kindle Font Scaling */
.font-scale-controls {
    display: none;
    align-items: center;
    gap: 0.25rem;
    border: 1px solid var(--border-color);
    border-radius: 8px;
    padding: 2px;
}

body.kindle-mode .font-scale-controls {
    display: flex;
    border: 2px solid #000000 !important;
    border-radius: 0px !important;
}

.btn-scale {
    background: transparent;
    border: none;
    padding: 0.25rem 0.5rem;
    cursor: pointer;
    font-family: var(--font-heading);
    font-weight: bold;
    font-size: 0.9rem;
}

.btn-scale:hover {
    background: #000000;
    color: #ffffff;
}

/* Animations (Modern Mode Only) */
@keyframes fadeIn {
    from { opacity: 0; transform: translateY(10px); }
    to { opacity: 1; transform: translateY(0); }
}

main.animated {
    animation: fadeIn 0.6s cubic-bezier(0.16, 1, 0.3, 1) forwards;
}

@media (prefers-reduced-motion: reduce) {
    *,
    *::before,
    *::after {
        animation-duration: 0.01ms !important;
        animation-iteration-count: 1 !important;
        scroll-behavior: auto !important;
        transition-duration: 0.01ms !important;
    }
}

/* Illustrations Styling */
.illustration-container {
    width: 100%;
    margin: 1.5rem 0;
    display: flex;
    justify-content: center;
    background-color: var(--bg-secondary);
    border: 1px solid var(--border-color);
    border-radius: 8px;
    padding: 1.5rem;
}

body.kindle-mode .illustration-container {
    background-color: #ffffff !important;
    border: 2px solid #000000 !important;
    border-radius: 0px !important;
}

.illustration {
    width: 100%;
    max-width: 580px;
    height: auto;
    display: block;
}

/* SVG Element styling matching the theme dynamically */
.illustration text {
    fill: var(--text-primary);
    font-family: var(--font-heading);
    font-weight: 500;
}

.illustration text.muted {
    fill: var(--text-muted);
}

.illustration .stroke-accent {
    stroke: var(--accent);
    stroke-width: 2px;
}

.illustration .fill-accent {
    fill: var(--accent);
}

.illustration .fill-accent-glow {
    fill: var(--accent-glow);
}

.illustration .stroke-border {
    stroke: var(--border-color);
    stroke-width: 1.5px;
}

.illustration .fill-card {
    fill: var(--bg-secondary);
}

.illustration .fill-tertiary {
    fill: var(--bg-tertiary);
}

.illustration .stroke-muted {
    stroke: var(--text-muted);
    stroke-dasharray: 4 4;
    stroke-width: 1.5px;
}

/* Kindle Mode overrides for SVGs to guarantee perfect high-contrast e-ink printing */
body.kindle-mode .illustration text {
    fill: #000000 !important;
    font-family: Georgia, serif !important;
}

body.kindle-mode .illustration text.muted {
    fill: #000000 !important;
}

body.kindle-mode .illustration .stroke-accent {
    stroke: #000000 !important;
    stroke-width: 2px !important;
}

body.kindle-mode .illustration .fill-accent {
    fill: #000000 !important;
}

body.kindle-mode .illustration .fill-accent-glow {
    fill: transparent !important;
}

body.kindle-mode .illustration .stroke-border {
    stroke: #000000 !important;
    stroke-width: 2px !important;
}

body.kindle-mode .illustration .fill-card {
    fill: #ffffff !important;
}

body.kindle-mode .illustration .fill-tertiary {
    fill: #ffffff !important;
    stroke: #000000 !important;
    stroke-width: 1px !important;
}

body.kindle-mode .illustration .stroke-muted {
    stroke: #000000 !important;
    stroke-dasharray: 2 2 !important;
}

/* ==========================================================================
   Responsive adjustments
   ========================================================================== */

@media (max-width: 600px) {
    body {
        font-size: calc(16px * var(--font-scale, 1));
    }
    
    h1 { font-size: 1.8rem; }
    h2 { font-size: 1.4rem; }
    
    .hero {
        padding: 2rem 1rem;
        margin-bottom: 2rem;
    }
    
    .hero h1 {
        font-size: 2.2rem;
    }
    
    .chapter-card {
        flex-direction: column;
        gap: 0.75rem;
        padding: 1.25rem;
    }
    
    .chapter-number {
        text-align: left;
        font-size: 1.5rem;
    }
    
    .chapter-navigation {
        flex-direction: column;
        gap: 1.5rem;
        align-items: stretch;
    }
    
    .nav-link-card {
        max-width: 100%;
    }
    
    .nav-link-card.next {
        text-align: left;
        align-items: flex-start;
    }
}