:root {
    --primary: #0066cc;
    --primary-dark: #004499;
    --secondary: #6c757d;
    --accent: #ff7f0e;
    --success: #2ca02c;
    --background: #f5f5f5;
    --surface: #ffffff;
    --text: #1a1a1a;
    --text-light: #444444;
    --text-muted: #666666;
    --heading: #000000;
    --border: #ddd;
    --lavender: #adadd0;
    --lavender-light: #c1c1e8;
    /* Section colors */
    --section-blue: #e8f4fc;
    --section-green: #e8f8e8;
    --section-yellow: #fef9e7;
    --section-gray: #f8f8f8;
    /* Difficulty badges */
    --badge-beginner: #27ae60;
    --badge-intermediate: #f39c12;
    --badge-advanced: #e74c3c;
}

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

body {
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
    line-height: 1.25;
    color: var(--text);
    background: var(--background);
    font-size: 13px;
}

.container { max-width: 1200px; margin: 0 auto; padding: 0 0.5rem; }

/* Two-Column Layout */
.site-layout {
    display: grid;
    grid-template-columns: 180px 1fr;
    gap: 0.5rem;
    max-width: 1400px;
    margin: 0 auto;
    padding: 0 0.5rem;
}

/* Sidebar */
.sidebar {
    position: sticky;
    top: 0.25rem;
    height: fit-content;
    max-height: calc(100vh - 0.5rem);
    overflow-y: auto;
    background: var(--surface);
    padding: 0.4rem;
    border-radius: 4px;
    font-size: 11px;
    box-shadow: 0 1px 2px rgba(0,0,0,0.08);
}

.sidebar-nav .nav-section { margin-bottom: 0.4rem; }

.sidebar-nav .nav-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    font-size: 9px;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.3px;
    color: var(--text-light);
    margin-bottom: 0.15rem;
    padding: 0.15rem 0.2rem;
    border-bottom: 1px solid var(--border);
    cursor: pointer;
    user-select: none;
}

.sidebar-nav .nav-header:hover { background: var(--background); }

.sidebar-nav .nav-header .icon { margin-right: 0.3rem; font-size: 10px; }
.sidebar-nav .nav-header .count { font-weight: 400; color: var(--text-muted); }
.sidebar-nav .nav-header .chevron { transition: transform 0.2s; font-size: 8px; }
.sidebar-nav .nav-section.collapsed .chevron { transform: rotate(-90deg); }
.sidebar-nav .nav-section.collapsed ul { display: none; }

/* Main navigation in sidebar */
.sidebar-nav .nav-main {
    background: var(--primary);
    margin: -0.4rem -0.4rem 0.5rem -0.4rem;
    padding: 0.3rem;
    border-radius: 4px 4px 0 0;
}
.sidebar-nav .nav-main ul { display: flex; flex-wrap: wrap; gap: 0.2rem; }
.sidebar-nav .nav-main a {
    color: rgba(255,255,255,0.9);
    font-weight: 600;
    font-size: 10px;
    padding: 0.2rem 0.4rem;
    border-radius: 3px;
}
.sidebar-nav .nav-main a:hover { background: rgba(255,255,255,0.2); color: white; }
.sidebar-nav .nav-main a.active { background: white; color: var(--primary); }

.sidebar-nav ul { list-style: none; margin: 0; padding: 0; }
.sidebar-nav li { margin: 0; }

.sidebar-nav a {
    display: block;
    padding: 0.15rem 0.3rem;
    color: var(--text);
    text-decoration: none;
    border-radius: 2px;
    line-height: 1.2;
}

.sidebar-nav a:hover { background: var(--background); color: var(--primary); }
.sidebar-nav a.active { background: var(--primary); color: white; }

.main-content { min-width: 0; }

/* Header - Minimal */
.site-header {
    background: linear-gradient(135deg, var(--primary) 0%, var(--primary-dark) 100%);
    color: white;
    padding: 0.25rem 0;
}

.site-header .container { text-align: center; }

.site-title { font-size: 0.9rem; font-weight: 700; color: white; text-decoration: none; }

/* Main */
.site-main { min-height: calc(100vh - 100px); padding: 0.5rem 0; }

/* Typography - Higher Contrast */
h1, h2, h3, h4, h5, h6 { color: var(--heading); font-weight: 600; }
h1 { font-size: 1.3rem; line-height: 1.15; margin-bottom: 0.3rem; }
h2 { font-size: 1.1rem; line-height: 1.15; margin: 0.5rem 0 0.2rem; border-bottom: 1px solid var(--lavender); padding-bottom: 0.15rem; }
h3 { font-size: 0.95rem; line-height: 1.2; margin: 0.4rem 0 0.15rem; }
p { margin-bottom: 0.3rem; }
a { color: var(--primary); text-decoration: none; }
a:hover { text-decoration: underline; }

/* Focus */
a:focus-visible, button:focus-visible { outline: 2px solid var(--accent); outline-offset: 1px; }
.sr-only { position: absolute; width: 1px; height: 1px; padding: 0; margin: -1px; overflow: hidden; clip: rect(0,0,0,0); border: 0; }

/* Tables */
table { width: 100%; border-collapse: collapse; margin: 0.3rem 0; background: var(--surface); font-size: 0.8rem; }
th, td { padding: 0.25rem 0.4rem; text-align: left; border-bottom: 1px solid var(--border); }
th { background: var(--lavender-light); font-weight: 600; color: var(--heading); }

/* Topic Cards - Compact with badges */
.topics-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(180px, 1fr)); gap: 0.35rem; margin: 0.4rem 0; }

.topic-card {
    background: var(--surface);
    border-radius: 4px;
    padding: 0.35rem;
    box-shadow: 0 1px 3px rgba(0,0,0,0.08);
    border-left: 2px solid var(--primary);
}

.topic-card:hover { box-shadow: 0 2px 6px rgba(0,0,0,0.12); }
.topic-card h3 { margin: 0 0 0.1rem 0; font-size: 0.8rem; color: var(--heading); }
.topic-card h3 a { color: var(--primary); }
.topic-card p { color: var(--text-light); font-size: 0.7rem; margin-bottom: 0.15rem; line-height: 1.2; }

.topic-card .card-meta {
    display: flex;
    gap: 0.3rem;
    align-items: center;
    flex-wrap: wrap;
    margin-top: 0.15rem;
}

.topic-card .badge {
    display: inline-block;
    padding: 0.08rem 0.25rem;
    border-radius: 2px;
    font-size: 0.6rem;
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 0.3px;
}

.topic-card .badge-beginner { background: var(--badge-beginner); color: white; }
.topic-card .badge-intermediate { background: var(--badge-intermediate); color: white; }
.topic-card .badge-advanced { background: var(--badge-advanced); color: white; }

.topic-card .duration {
    font-size: 0.6rem;
    color: var(--text-muted);
}

/* Lists */
ul, ol { margin: 0.25rem 0 0.25rem 1.2rem; }
li { margin-bottom: 0.1rem; }

/* Images */
img { max-width: 100%; height: auto; }

/* Blockquotes */
blockquote { border-left: 3px solid var(--lavender); padding: 0.3rem 0.5rem; margin: 0.3rem 0; background: var(--surface); color: var(--text-light); font-style: italic; }
blockquote p:last-child { margin-bottom: 0; }

/* Code */
code { background: var(--lavender-light); padding: 0.1rem 0.25rem; border-radius: 2px; font-family: "SF Mono", Monaco, monospace; font-size: 0.8em; }
pre { background: #2d2d2d; color: #f8f8f2; padding: 0.4rem; border-radius: 4px; overflow-x: auto; margin: 0.3rem 0; font-size: 0.75rem; }
pre code { background: none; padding: 0; color: inherit; }

/* Topic Page Navigation - styled */
.page-nav {
    display: flex;
    justify-content: space-between;
    gap: 0.5rem;
    margin-top: 0.5rem;
    padding-top: 0.3rem;
    border-top: 2px solid var(--primary);
    background: var(--section-gray);
    padding: 0.4rem;
    border-radius: 4px;
}
.page-nav a {
    color: var(--primary);
    font-size: 0.8rem;
    font-weight: 500;
    padding: 0.2rem 0.4rem;
    border-radius: 3px;
    background: var(--surface);
}
.page-nav a:hover { background: var(--primary); color: white; text-decoration: none; }

/* Download Button */
.download-btn {
    display: inline-flex;
    align-items: center;
    gap: 0.3rem;
    background: var(--accent);
    color: white;
    padding: 0.35rem 0.7rem;
    border-radius: 3px;
    text-decoration: none;
    font-weight: 600;
    font-size: 0.8rem;
    margin: 0.2rem 0;
    box-shadow: 0 2px 4px rgba(255,127,14,0.3);
}
.download-btn:hover { background: #e06f00; text-decoration: none; color: white; }

/* Download All Buttons Container */
.download-all-buttons {
    display: flex;
    flex-wrap: wrap;
    gap: 0.5rem;
    margin: 0.4rem 0;
}
.download-all-buttons .download-btn { margin: 0; }

/* Footer */
.site-footer { background: var(--primary-dark); color: rgba(255,255,255,0.9); padding: 0.4rem 0; text-align: center; font-size: 0.75rem; }
.site-footer a { color: var(--lavender-light); }
.site-footer p { margin: 0.1rem 0; }

/* Hero - Compact */
.hero {
    background: linear-gradient(135deg, var(--lavender) 0%, var(--lavender-light) 100%);
    padding: 0.35rem 0.5rem;
    border-radius: 4px;
    margin-bottom: 0.3rem;
    text-align: center;
}
.hero h1 { color: var(--heading); margin-bottom: 0.1rem; font-size: 1rem; }
.hero p { font-size: 0.75rem; color: var(--text); margin: 0 auto; }

/* Homepage Navigation Bar */
.home-nav {
    display: flex;
    justify-content: center;
    gap: 0.5rem;
    margin-bottom: 0.4rem;
    padding: 0.3rem;
    background: var(--primary);
    border-radius: 4px;
}
.home-nav a {
    color: white;
    text-decoration: none;
    font-weight: 600;
    font-size: 0.8rem;
    padding: 0.25rem 0.6rem;
    border-radius: 3px;
    transition: background 0.2s;
}
.home-nav a:hover { background: rgba(255,255,255,0.2); text-decoration: none; }

/* Metadata */
.topic-meta { display: flex; flex-wrap: wrap; gap: 0.5rem; margin: 0.3rem 0; padding: 0.3rem; background: var(--surface); border-radius: 3px; font-size: 0.75rem; border: 1px solid var(--border); }
.topic-meta span { display: flex; align-items: center; gap: 0.2rem; }
.topic-meta strong { color: var(--heading); }

/* Chart Grid - 3 columns */
.chart-grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 0.6rem;
    margin: 0.5rem 0;
}
.chart-item {
    background: var(--surface);
    border-radius: 4px;
    padding: 0.25rem;
    box-shadow: 0 1px 3px rgba(0,0,0,0.1);
    text-align: center;
    cursor: pointer;
    transition: transform 0.2s, box-shadow 0.2s;
}
.chart-item:hover { transform: scale(1.03); box-shadow: 0 4px 12px rgba(0,0,0,0.15); }
.chart-item a { display: block; }
.chart-item img { width: 100%; height: auto; border-radius: 3px; }
.chart-caption { font-size: 10px; color: var(--text-light); margin-top: 0.2rem; font-weight: 500; line-height: 1.2; }

/* Visual Sections for Topic Pages */
.topic-content h2[id*="key"], .topic-content h2:first-of-type { margin-top: 0; }

/* Section backgrounds */
article h2 + ul, article h2 + p, article h2 + table,
article h2 + ol, article h2 + blockquote {
    background: var(--section-gray);
    padding: 0.3rem;
    border-radius: 4px;
    margin-bottom: 0.3rem;
}

/* Key Concepts - blue */
article h2:has(+ *):nth-of-type(2) + *,
article h2:contains("Key") + *,
article h2:contains("Concepts") + * {
    background: var(--section-blue);
}

/* Visual sections by heading text - using adjacent sibling */
article section.key-concepts { background: var(--section-blue); padding: 0.4rem; border-radius: 4px; margin: 0.3rem 0; }
article section.when-to-use { background: var(--section-green); padding: 0.4rem; border-radius: 4px; margin: 0.3rem 0; }
article section.pitfalls { background: var(--section-yellow); padding: 0.4rem; border-radius: 4px; margin: 0.3rem 0; }

/* Lightbox Modal */
.lightbox {
    display: none;
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(0,0,0,0.9);
    z-index: 1000;
    justify-content: center;
    align-items: center;
    cursor: zoom-out;
}
.lightbox.active { display: flex; }
.lightbox img { max-width: 95%; max-height: 95%; object-fit: contain; }
.lightbox-close {
    position: absolute;
    top: 1rem;
    right: 1rem;
    color: white;
    font-size: 2rem;
    cursor: pointer;
    background: none;
    border: none;
}

/* Responsive */
@media (min-width: 1024px) { .topics-grid { grid-template-columns: repeat(4, 1fr); } }
@media (max-width: 900px) { .site-layout { grid-template-columns: 1fr; } .sidebar { display: none; } }
@media (max-width: 768px) {
    .site-header .container { flex-direction: column; text-align: center; gap: 0.25rem; }
    .main-nav { flex-wrap: wrap; justify-content: center; gap: 0.5rem; }
    h1 { font-size: 1.1rem; } h2 { font-size: 0.95rem; }
    .topics-grid { grid-template-columns: repeat(2, 1fr); }
    .chart-grid { grid-template-columns: repeat(2, 1fr); }
    .page-nav { flex-direction: column; gap: 0.25rem; }
}
@media (max-width: 500px) {
    .chart-grid { grid-template-columns: 1fr; }
}
@media (max-width: 480px) {
    body { font-size: 12px; }
    .container { padding: 0 0.4rem; }
    .topics-grid { grid-template-columns: 1fr; }
    .download-btn { width: 100%; justify-content: center; }
}

/* Print */
@media print { .site-header, .site-footer, .page-nav, .download-btn, .sidebar { display: none; } body { background: white; } }