/* ===== CSS Variables ===== */
:root {
    --color-bg: #fafbfc;
    --color-surface: #ffffff;
    --color-text: #24292f;
    --color-text-muted: #656d76;
    --color-accent: #0969da;
    --color-accent-hover: #0550ae;
    --color-accent-subtle: #ddf4ff;
    --color-border: #d0d7de;
    --color-border-light: #e8ebef;
    --color-code-bg: #f6f8fa;
    --font-sans: 'Source Sans 3', -apple-system, BlinkMacSystemFont, sans-serif;
    --font-mono: 'JetBrains Mono', SFMono-Regular, monospace;
    --max-width: 760px;
    --space-xs: 0.5rem;
    --space-sm: 1rem;
    --space-md: 1.5rem;
    --space-lg: 2rem;
    --space-xl: 3rem;
    --radius: 6px;
}

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

/* ===== Base Styles ===== */
html {
    font-size: 16px;
    -webkit-font-smoothing: antialiased;
}

body {
    font-family: var(--font-sans);
    background: var(--color-bg);
    color: var(--color-text);
    line-height: 1.6;
    min-height: 100vh;
    display: flex;
    flex-direction: column;
}

a {
    color: var(--color-accent);
    text-decoration: none;
    transition: color 0.15s ease;
}

a:hover {
    color: var(--color-accent-hover);
    text-decoration: underline;
}

img {
    max-width: 100%;
    height: auto;
    display: block;
}

/* ===== Layout ===== */
.container {
    max-width: var(--max-width);
    margin: 0 auto;
    padding: 0 var(--space-md);
}

.main {
    flex: 1;
    padding: var(--space-xl) 0;
}

/* ===== Navigation ===== */
.nav {
    background: var(--color-surface);
    border-bottom: 1px solid var(--color-border);
    position: sticky;
    top: 0;
    z-index: 100;
}

.nav-container {
    max-width: var(--max-width);
    margin: 0 auto;
    padding: 0 var(--space-md);
    display: flex;
    justify-content: space-between;
    align-items: center;
    height: 60px;
}

.nav-brand {
    font-weight: 600;
    font-size: 1.0625rem;
    color: var(--color-text);
}

.nav-brand:hover {
    color: var(--color-accent);
    text-decoration: none;
}

.nav-links {
    display: flex;
    gap: 0;
    height: 100%;
    align-items: center;
}

.nav-links a {
    color: var(--color-text-muted);
    font-size: 0.9rem;
    font-weight: 500;
    padding: 0 var(--space-sm);
    height: 100%;
    display: flex;
    align-items: center;
    position: relative;
    border-bottom: 2px solid transparent;
    margin-bottom: -1px;
    transition: all 0.15s ease;
}

.nav-links a:hover {
    color: var(--color-text);
    text-decoration: none;
    background: var(--color-bg);
}

.nav-links a.active {
    color: var(--color-text);
    border-bottom-color: var(--color-accent);
}

/* ===== Hero Section ===== */
.hero {
    padding: var(--space-xl) 0;
    border-bottom: 1px solid var(--color-border-light);
    margin-bottom: var(--space-xl);
}

.hero h1 {
    font-size: 1.875rem;
    font-weight: 600;
    margin-bottom: var(--space-xs);
    color: var(--color-text);
}

.hero-title {
    font-size: 1rem;
    color: var(--color-accent);
    font-family: var(--font-mono);
    margin-bottom: var(--space-md);
}

.hero-bio {
    color: var(--color-text-muted);
    line-height: 1.75;
    font-size: 1.0625rem;
}

/* ===== Page Header ===== */
.page-header {
    margin-bottom: var(--space-xl);
    padding-bottom: var(--space-lg);
    border-bottom: 1px solid var(--color-border-light);
}

.page-header h1 {
    font-size: 1.625rem;
    font-weight: 600;
    margin-bottom: var(--space-xs);
    color: var(--color-text);
}

.page-header p {
    color: var(--color-text-muted);
}

/* ===== Section ===== */
.section {
    margin-bottom: var(--space-xl);
}

.section-header {
    display: flex;
    justify-content: space-between;
    align-items: baseline;
    margin-bottom: var(--space-md);
    padding-bottom: var(--space-sm);
    border-bottom: 1px solid var(--color-border-light);
}

.section-header h2 {
    font-size: 0.875rem;
    font-weight: 600;
    color: var(--color-text);
    text-transform: uppercase;
    letter-spacing: 0.04em;
}

.link {
    font-size: 0.8125rem;
    color: var(--color-accent);
    font-weight: 500;
}

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

/* ===== Cards ===== */
.card-grid {
    display: grid;
    grid-template-columns: 1fr;
    gap: var(--space-sm);
}

.card {
    background: var(--color-surface);
    border: 1px solid var(--color-border-light);
    border-radius: var(--radius);
    padding: var(--space-md);
    position: relative;
    transition: border-color 0.15s ease, box-shadow 0.15s ease;
}

.card:hover {
    border-color: var(--color-border);
    box-shadow: 0 1px 3px rgba(0,0,0,0.04);
}

.card.featured {
    border-left: 3px solid var(--color-accent);
}

.featured-badge {
    position: absolute;
    top: var(--space-sm);
    right: var(--space-sm);
    background: var(--color-accent-subtle);
    color: var(--color-accent);
    font-size: 0.6875rem;
    font-weight: 600;
    padding: 0.25rem 0.5rem;
    border-radius: var(--radius);
    text-transform: uppercase;
    letter-spacing: 0.03em;
}

.card h2, .card h3 {
    font-size: 1rem;
    font-weight: 600;
    margin-bottom: var(--space-xs);
    color: var(--color-text);
}

.card p {
    color: var(--color-text-muted);
    font-size: 0.9375rem;
    margin-bottom: var(--space-sm);
    line-height: 1.6;
}

.card-links {
    display: flex;
    gap: var(--space-sm);
}

.card-links a {
    font-size: 0.8125rem;
    font-weight: 500;
}

/* ===== Tags ===== */
.tags {
    display: flex;
    flex-wrap: wrap;
    gap: var(--space-xs);
}

.tag {
    background: var(--color-accent-subtle);
    color: var(--color-accent);
    font-size: 0.75rem;
    font-family: var(--font-mono);
    padding: 0.2rem 0.5rem;
    border-radius: var(--radius);
}

/* ===== Blog List ===== */
.blog-list {
    display: flex;
    flex-direction: column;
}

.blog-item {
    padding: var(--space-md) 0;
    border-bottom: 1px solid var(--color-border-light);
}

.blog-item:first-child {
    padding-top: 0;
}

.blog-item:last-child {
    border-bottom: none;
}

.blog-item time {
    font-size: 0.8125rem;
    color: var(--color-text-muted);
    font-family: var(--font-mono);
}

.blog-item h2, .blog-item h3 {
    font-size: 1.125rem;
    font-weight: 600;
    margin: var(--space-xs) 0;
}

.blog-item h2 a, .blog-item h3 a {
    color: var(--color-text);
}

.blog-item h2 a:hover, .blog-item h3 a:hover {
    color: var(--color-accent);
    text-decoration: none;
}

.blog-item p {
    color: var(--color-text-muted);
    line-height: 1.65;
}

/* ===== Blog Detail ===== */
.blog-header {
    margin-bottom: var(--space-xl);
}

.back-link {
    display: inline-flex;
    align-items: center;
    gap: 0.375rem;
    font-size: 0.875rem;
    color: var(--color-text-muted);
    margin-bottom: var(--space-md);
}

.back-link:hover {
    color: var(--color-accent);
    text-decoration: none;
}

.blog-header h1 {
    font-size: 1.75rem;
    font-weight: 600;
    margin-bottom: var(--space-xs);
    color: var(--color-text);
}

.blog-header time {
    color: var(--color-text-muted);
    font-family: var(--font-mono);
    font-size: 0.875rem;
}

/* ===== Prose (Blog Content) ===== */
.prose {
    line-height: 1.75;
    color: var(--color-text);
}

.prose h1, .prose h2, .prose h3, .prose h4 {
    margin-top: var(--space-lg);
    margin-bottom: var(--space-sm);
    font-weight: 600;
    color: var(--color-text);
}

.prose h2 { font-size: 1.375rem; }
.prose h3 { font-size: 1.125rem; }
.prose h4 { font-size: 1rem; }

.prose p {
    margin-bottom: var(--space-md);
}

.prose ul, .prose ol {
    margin-bottom: var(--space-md);
    padding-left: var(--space-md);
}

.prose li {
    margin-bottom: var(--space-xs);
}

.prose code {
    background: var(--color-code-bg);
    font-family: var(--font-mono);
    font-size: 0.875em;
    padding: 0.2em 0.4em;
    border-radius: var(--radius);
    color: var(--color-text);
}

.prose pre {
    background: var(--color-code-bg);
    padding: var(--space-md);
    border-radius: var(--radius);
    overflow-x: auto;
    margin-bottom: var(--space-md);
    border: 1px solid var(--color-border-light);
}

.prose pre code {
    background: none;
    padding: 0;
}

.prose blockquote {
    border-left: 3px solid var(--color-accent);
    padding-left: var(--space-md);
    color: var(--color-text-muted);
    font-style: italic;
    margin-bottom: var(--space-md);
}

.prose a {
    text-decoration: underline;
    text-underline-offset: 2px;
}

.prose img {
    border-radius: var(--radius);
    margin: var(--space-md) 0;
}

/* ===== White Papers ===== */
.paper-list {
    display: flex;
    flex-direction: column;
}

.paper-item {
    display: flex;
    justify-content: space-between;
    gap: var(--space-md);
    padding: var(--space-md) 0;
    border-bottom: 1px solid var(--color-border-light);
}

.paper-item:first-child {
    padding-top: 0;
}

.paper-item:last-child {
    border-bottom: none;
}

.paper-main {
    flex: 1;
}

.paper-item h2 {
    font-size: 1rem;
    font-weight: 600;
    margin-bottom: var(--space-xs);
}

.paper-item h2 a {
    color: var(--color-text);
}

.paper-item h2 a:hover {
    color: var(--color-accent);
    text-decoration: none;
}

.paper-authors {
    font-size: 0.8125rem;
    color: var(--color-text-muted);
    margin-bottom: var(--space-xs);
}

.paper-summary {
    color: var(--color-text-muted);
    font-size: 0.9375rem;
    line-height: 1.65;
    margin-bottom: var(--space-sm);
}

.paper-date {
    font-size: 0.8125rem;
    color: var(--color-text-muted);
    font-family: var(--font-mono);
    white-space: nowrap;
}

/* ===== Books ===== */
.book-list {
    display: flex;
    flex-direction: column;
}

.book-item {
    display: flex;
    justify-content: space-between;
    gap: var(--space-md);
    padding: var(--space-md) 0;
    border-bottom: 1px solid var(--color-border-light);
}

.book-item:first-child {
    padding-top: 0;
}

.book-item:last-child {
    border-bottom: none;
}

.book-main {
    flex: 1;
}

.book-item h2 {
    font-size: 1rem;
    font-weight: 600;
    margin-bottom: var(--space-xs);
}

.book-item h2 a {
    color: var(--color-text);
}

.book-item h2 a:hover {
    color: var(--color-accent);
    text-decoration: none;
}

.book-author {
    font-size: 0.8125rem;
    color: var(--color-text-muted);
    margin-bottom: var(--space-xs);
}

.book-rating {
    color: #d97706;
    font-size: 0.875rem;
    margin-bottom: var(--space-xs);
}

.book-summary {
    color: var(--color-text-muted);
    font-size: 0.9375rem;
    line-height: 1.65;
    margin-bottom: var(--space-sm);
}

.book-date {
    font-size: 0.8125rem;
    color: var(--color-text-muted);
    font-family: var(--font-mono);
    white-space: nowrap;
}

/* ===== YouTube Tutorials ===== */
.tutorial-grid, .tutorial-grid-full {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(260px, 1fr));
    gap: var(--space-md);
}

.tutorial-card {
    display: block;
    color: var(--color-text);
    background: var(--color-surface);
    border: 1px solid var(--color-border-light);
    border-radius: var(--radius);
    overflow: hidden;
    transition: border-color 0.15s ease, box-shadow 0.15s ease;
}

.tutorial-card:hover {
    color: var(--color-text);
    text-decoration: none;
    border-color: var(--color-border);
    box-shadow: 0 2px 8px rgba(0,0,0,0.06);
}

.tutorial-thumb {
    position: relative;
    aspect-ratio: 16 / 9;
    background: var(--color-code-bg);
    overflow: hidden;
}

.tutorial-thumb img {
    width: 100%;
    height: 100%;
    object-fit: cover;
}

.play-icon {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    width: 44px;
    height: 44px;
    background: rgba(0, 0, 0, 0.7);
    color: white;
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 0.875rem;
    opacity: 0;
    transition: opacity 0.15s ease;
}

.tutorial-card:hover .play-icon {
    opacity: 1;
}

.tutorial-info {
    padding: var(--space-sm);
}

.tutorial-info h2 {
    font-size: 0.9375rem;
    font-weight: 600;
    margin-bottom: var(--space-xs);
    color: var(--color-text);
    line-height: 1.4;
}

.tutorial-info p {
    font-size: 0.8125rem;
    color: var(--color-text-muted);
    margin-bottom: var(--space-xs);
    line-height: 1.5;
}

.tutorial-card h3 {
    font-size: 0.9375rem;
    font-weight: 600;
    color: var(--color-text);
    padding: var(--space-sm);
}

/* ===== Pagination ===== */
.pagination {
    display: flex;
    justify-content: center;
    align-items: center;
    gap: var(--space-md);
    margin-top: var(--space-xl);
    padding-top: var(--space-lg);
    border-top: 1px solid var(--color-border-light);
}

.pagination span {
    color: var(--color-text-muted);
    font-size: 0.875rem;
}

.pagination a {
    font-weight: 500;
}

/* ===== Footer ===== */
.footer {
    background: var(--color-surface);
    border-top: 1px solid var(--color-border);
    padding: var(--space-lg) 0;
    margin-top: auto;
}

.footer .container {
    text-align: center;
}

.footer-links {
    display: flex;
    justify-content: center;
    gap: var(--space-md);
    margin-bottom: var(--space-sm);
}

.footer-links a {
    color: var(--color-text-muted);
    font-size: 0.875rem;
}

.footer-links a:hover {
    color: var(--color-accent);
}

.footer p {
    color: var(--color-text-muted);
    font-size: 0.8125rem;
}

/* ===== Empty State ===== */
.empty-state {
    color: var(--color-text-muted);
    font-style: italic;
    padding: var(--space-xl) 0;
    text-align: center;
}

/* ===== Contact ===== */
.contact-page {
    max-width: 480px;
}

.contact-intro {
    color: var(--color-text-muted);
    line-height: 1.7;
    margin-bottom: var(--space-lg);
}

.contact-methods {
    display: flex;
    flex-direction: column;
}

.contact-item {
    display: flex;
    align-items: center;
    gap: var(--space-md);
    padding: var(--space-md);
    margin: 0 calc(-1 * var(--space-md));
    border-radius: var(--radius);
    color: var(--color-text);
    transition: background 0.15s ease;
}

.contact-item:hover {
    text-decoration: none;
    background: var(--color-surface);
}

.contact-icon {
    font-size: 1.25rem;
    width: 2.5rem;
    height: 2.5rem;
    display: flex;
    align-items: center;
    justify-content: center;
    background: var(--color-accent-subtle);
    border-radius: var(--radius);
}

.contact-details h3 {
    font-size: 0.9375rem;
    font-weight: 600;
    color: var(--color-text);
    margin-bottom: 0.125rem;
}

.contact-details span {
    font-size: 0.8125rem;
    color: var(--color-text-muted);
}

.contact-item:hover .contact-details h3 {
    color: var(--color-accent);
}

/* ===== Open Source ===== */
.opensource-intro {
    color: var(--color-text-muted);
    line-height: 1.7;
    margin-bottom: var(--space-lg);
}

.contribution-list {
    display: flex;
    flex-direction: column;
}

.contribution-item {
    display: flex;
    align-items: flex-start;
    gap: var(--space-md);
    padding: var(--space-md);
    margin: 0 calc(-1 * var(--space-md));
    border-radius: var(--radius);
    color: var(--color-text);
    transition: background 0.15s ease;
}

.contribution-item:hover {
    text-decoration: none;
    background: var(--color-surface);
}

.contribution-icon {
    font-size: 1.25rem;
    width: 2.5rem;
    height: 2.5rem;
    display: flex;
    align-items: center;
    justify-content: center;
    background: var(--color-accent-subtle);
    border-radius: var(--radius);
    flex-shrink: 0;
}

.contribution-details {
    flex: 1;
    min-width: 0;
}

.contribution-details h3 {
    font-size: 1rem;
    font-weight: 600;
    color: var(--color-text);
    margin-bottom: 0.25rem;
}

.contribution-details p {
    font-size: 0.875rem;
    color: var(--color-text-muted);
    line-height: 1.5;
}

.contribution-item:hover .contribution-details h3 {
    color: var(--color-accent);
}

.contribution-arrow {
    color: var(--color-text-muted);
    font-size: 1rem;
    padding-top: 0.625rem;
    transition: transform 0.15s ease;
}

.contribution-item:hover .contribution-arrow {
    transform: translateX(3px);
    color: var(--color-accent);
}

/* ===== Responsive ===== */
@media (max-width: 640px) {
    .nav-container {
        height: auto;
        flex-direction: column;
        padding: var(--space-sm) var(--space-md);
        gap: var(--space-sm);
    }
    
    .nav-links {
        height: auto;
        flex-wrap: wrap;
        justify-content: center;
    }
    
    .nav-links a {
        height: auto;
        padding: var(--space-xs) var(--space-sm);
        border-bottom: none;
        border-radius: var(--radius);
    }
    
    .nav-links a.active {
        background: var(--color-accent-subtle);
        border-bottom-color: transparent;
    }
    
    .hero h1 {
        font-size: 1.5rem;
    }
    
    .paper-item, .book-item {
        flex-direction: column;
    }
    
    .paper-date, .book-date {
        order: -1;
    }
    
    .section-header {
        flex-direction: column;
        gap: var(--space-xs);
        align-items: flex-start;
    }
    
    .tutorial-grid, .tutorial-grid-full {
        grid-template-columns: 1fr;
    }
    
    .contact-item, .contribution-item {
        margin: 0;
    }
}