:root {
    --bg: #ffffff;
    --bg-sidebar: #f8f9fa;
    --bg-code: #1e1e2e;
    --bg-inline-code: #f1f3f5;
    --bg-hover: rgba(0,0,0,0.03);
    --text: #1a1a1a;
    --text-secondary: #4a4a4a;
    --text-muted: #6c757d;
    --text-code: #c7254e;
    --border: #e5e7eb;
    --border-dark: #d1d5db;
    --accent: #2563eb;
    --accent-hover: #1d4ed8;
    --accent-light: #eff6ff;
    --green-bg: #f0fdf4;
    --green-border: #bbf7d0;
    --green-text: #166534;
    --yellow-bg: #fffbeb;
    --yellow-border: #fde68a;
    --yellow-text: #92400e;
    --blue-bg: #eff6ff;
    --blue-border: #bfdbfe;
    --font-sans: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', sans-serif;
    --font-mono: 'SFMono-Regular', Consolas, 'Liberation Mono', Menlo, monospace;
    --max-width: 960px;
    --sidebar-width: 260px;
    --code-text: #d4d4d8;
    --code-keyword: #569cd6;
    --code-string: #ce9178;
    --code-comment: #6a9955;
    --code-number: #b5cea8;
    --code-builtin: #dcdcaa;
}

[data-theme="dark"] {
    --bg: #0f1117;
    --bg-sidebar: #161822;
    --bg-inline-code: #1e2030;
    --bg-hover: rgba(255,255,255,0.04);
    --text: #e0e0e0;
    --text-secondary: #a0a0a0;
    --text-muted: #6c757d;
    --text-code: #f97583;
    --border: #2a2d3a;
    --border-dark: #3a3d4a;
    --accent: #5b9af5;
    --accent-hover: #7db4ff;
    --accent-light: rgba(91,154,245,0.1);
    --green-bg: rgba(22,163,74,0.1);
    --green-border: rgba(22,163,74,0.3);
    --green-text: #4ade80;
    --yellow-bg: rgba(217,119,6,0.1);
    --yellow-border: rgba(217,119,6,0.3);
    --yellow-text: #fbbf24;
    --blue-bg: rgba(91,154,245,0.1);
    --blue-border: rgba(91,154,245,0.3);
    --code-text: #d4d4d8;
    --code-keyword: #569cd6;
    --code-string: #ce9178;
    --code-comment: #6a9955;
}

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

body {
    font-family: var(--font-sans);
    font-size: 16px;
    line-height: 1.7;
    color: var(--text);
    background: var(--bg);
    -webkit-font-smoothing: antialiased;
    transition: background 0.2s, color 0.2s;
}

a { color: var(--accent); text-decoration: none; }
a:hover { color: var(--accent-hover); text-decoration: underline; }

code {
    font-family: var(--font-mono);
    background: var(--bg-inline-code);
    padding: 2px 6px;
    border-radius: 4px;
    font-size: 0.875em;
    color: var(--text-code);
}

/* Code blocks with copy button */
.code-block {
    position: relative;
    margin: 16px 0;
}

.code-block pre {
    background: var(--bg-code);
    color: var(--code-text);
    padding: 16px 20px;
    border-radius: 6px;
    overflow-x: auto;
    font-family: var(--font-mono);
    font-size: 0.875rem;
    line-height: 1.6;
    border: 1px solid var(--border);
}

.code-block pre code {
    background: none;
    padding: 0;
    color: inherit;
    font-size: inherit;
    font-family: inherit;
}

.code-block .copy-btn {
    position: absolute;
    top: 8px;
    right: 8px;
    background: rgba(255,255,255,0.1);
    border: 1px solid rgba(255,255,255,0.15);
    color: rgba(255,255,255,0.7);
    padding: 4px 10px;
    border-radius: 4px;
    font-size: 0.75rem;
    font-family: var(--font-sans);
    cursor: pointer;
    opacity: 0;
    transition: opacity 0.2s, background 0.2s;
}

.code-block:hover .copy-btn {
    opacity: 1;
}

.code-block .copy-btn:hover {
    background: rgba(255,255,255,0.2);
    color: #fff;
}

.code-block .copy-btn.copied {
    background: rgba(22,163,74,0.3);
    color: #4ade80;
    border-color: rgba(22,163,74,0.4);
}

/* Standalone pre without copy (backward compat) */
pre {
    background: var(--bg-code);
    color: var(--code-text);
    padding: 16px 20px;
    border-radius: 6px;
    overflow-x: auto;
    margin: 16px 0;
    font-family: var(--font-mono);
    font-size: 0.875rem;
    line-height: 1.6;
    border: 1px solid var(--border);
}

pre code {
    background: none;
    padding: 0;
    color: inherit;
    font-size: inherit;
}

blockquote {
    border-left: 3px solid var(--accent);
    padding: 8px 20px;
    margin: 16px 0;
    background: var(--accent-light);
    color: var(--text-secondary);
    font-style: italic;
}

blockquote em { font-style: normal; color: var(--text); }

table {
    width: 100%;
    border-collapse: collapse;
    margin: 16px 0;
    font-size: 0.9rem;
}

th, td {
    padding: 10px 14px;
    border: 1px solid var(--border);
    text-align: left;
}

th { background: var(--bg-sidebar); font-weight: 600; }

h1 { font-size: 2rem; font-weight: 700; margin: 48px 0 16px; padding-bottom: 8px; border-bottom: 1px solid var(--border); }
h2 { font-size: 1.5rem; font-weight: 600; margin: 40px 0 12px; }
h3 { font-size: 1.15rem; font-weight: 600; margin: 28px 0 8px; }
h4 { font-size: 1rem; font-weight: 600; margin: 20px 0 6px; }

p { margin-bottom: 12px; }

ul, ol { margin: 8px 0 16px 24px; }
li { margin-bottom: 6px; }

/* Layout */
.layout {
    display: flex;
    min-height: 100vh;
}

.sidebar {
    width: var(--sidebar-width);
    background: var(--bg-sidebar);
    border-right: 1px solid var(--border);
    padding: 24px 0;
    position: fixed;
    top: 0;
    bottom: 0;
    overflow-y: auto;
    z-index: 50;
    transition: background 0.2s, border-color 0.2s;
}

.sidebar-brand {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 0 20px 20px;
    font-size: 1.1rem;
    font-weight: 700;
    color: var(--text);
    border-bottom: 1px solid var(--border);
    margin-bottom: 16px;
    text-decoration: none;
}

.sidebar-brand:hover { text-decoration: none; }

.theme-toggle {
    background: none;
    border: 1px solid var(--border);
    border-radius: 4px;
    padding: 4px 8px;
    font-size: 0.85rem;
    cursor: pointer;
    color: var(--text-secondary);
    line-height: 1;
    transition: background 0.2s, border-color 0.2s;
}

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

.sidebar-section {
    margin-bottom: 8px;
}

.sidebar-heading {
    padding: 8px 20px 4px;
    font-size: 0.7rem;
    font-weight: 600;
    text-transform: uppercase;
    letter-spacing: 1px;
    color: var(--text-muted);
}

.sidebar-link {
    display: block;
    padding: 6px 20px;
    color: var(--text-secondary);
    font-size: 0.9rem;
    text-decoration: none;
    border-left: 3px solid transparent;
    transition: color 0.15s, background 0.15s, border-color 0.15s;
}

.sidebar-link:hover {
    color: var(--text);
    background: var(--bg-hover);
    text-decoration: none;
}

.sidebar-link.active {
    color: var(--accent);
    border-left-color: var(--accent);
    background: var(--accent-light);
    font-weight: 500;
}

/* Collapsible sections */
details {
    border: 1px solid var(--border);
    border-radius: 6px;
    margin: 12px 0;
    background: var(--bg);
    transition: background 0.2s, border-color 0.2s;
}

details summary {
    padding: 12px 16px;
    cursor: pointer;
    font-weight: 600;
    font-size: 0.9rem;
    list-style: none;
    display: flex;
    align-items: center;
    gap: 8px;
    user-select: none;
    transition: color 0.15s;
}

details summary::-webkit-details-marker { display: none; }

details summary::before {
    content: '';
    display: inline-block;
    width: 0;
    height: 0;
    border-left: 5px solid var(--text-muted);
    border-top: 4px solid transparent;
    border-bottom: 4px solid transparent;
    transition: transform 0.2s;
    flex-shrink: 0;
}

details[open] summary::before {
    transform: rotate(90deg);
}

details summary:hover {
    color: var(--accent);
}

details[open] summary {
    border-bottom: 1px solid var(--border);
}

details > div,
details > p,
details > .details-content {
    padding: 12px 16px;
}

/* Sidebar collapsible */
.sidebar details {
    border: none;
    border-radius: 0;
    margin: 0;
    background: none;
}

.sidebar details summary {
    padding: 4px 20px;
    font-weight: 400;
    font-size: 0.9rem;
    color: var(--text-secondary);
}

.sidebar details summary::before {
    border-left-color: var(--text-muted);
    width: 0;
    height: 0;
    border-left: 4px solid currentColor;
    border-top: 3px solid transparent;
    border-bottom: 3px solid transparent;
}

.sidebar details summary:hover {
    color: var(--accent);
    background: var(--bg-hover);
}

.sidebar details[open] summary {
    color: var(--text);
}

.content {
    flex: 1;
    margin-left: var(--sidebar-width);
    padding: 40px 48px 80px;
    max-width: calc(var(--max-width) + var(--sidebar-width));
}

.content-inner {
    max-width: var(--max-width);
}

/* Top bar for mobile */
.topbar {
    display: none;
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    height: 56px;
    background: var(--bg);
    border-bottom: 1px solid var(--border);
    z-index: 60;
    padding: 0 16px;
    align-items: center;
    justify-content: space-between;
    transition: background 0.2s, border-color 0.2s;
}

.topbar-brand {
    font-size: 1rem;
    font-weight: 700;
    color: var(--text);
    text-decoration: none;
}

.topbar-actions {
    display: flex;
    align-items: center;
    gap: 8px;
}

.topbar-toggle {
    background: none;
    border: 1px solid var(--border);
    border-radius: 4px;
    padding: 6px 10px;
    font-size: 0.85rem;
    cursor: pointer;
    color: var(--text-secondary);
}

/* Problem meta */
.problem-meta {
    display: flex;
    gap: 16px;
    margin-bottom: 24px;
    flex-wrap: wrap;
}

.meta-tag {
    font-size: 0.8rem;
    padding: 3px 10px;
    border-radius: 3px;
    font-weight: 500;
}

.meta-tag.beginner { background: #dcfce7; color: #166534; }
.meta-tag.intermediate { background: #fef3c7; color: #92400e; }
[data-theme="dark"] .meta-tag.beginner { background: rgba(22,163,74,0.15); color: #4ade80; }
[data-theme="dark"] .meta-tag.intermediate { background: rgba(217,119,6,0.15); color: #fbbf24; }

.meta-info {
    font-size: 0.85rem;
    color: var(--text-muted);
}

/* Tip/lesson boxes */
.tip-box {
    background: var(--yellow-bg);
    border: 1px solid var(--yellow-border);
    border-radius: 6px;
    padding: 16px 20px;
    margin: 20px 0;
}

.tip-box h4 { color: var(--yellow-text); margin: 0 0 6px; font-size: 0.9rem; }
.tip-box p { color: var(--text-secondary); font-size: 0.9rem; margin: 0; }

.lesson-box {
    background: var(--green-bg);
    border: 1px solid var(--green-border);
    border-radius: 6px;
    padding: 16px 20px;
    margin: 20px 0;
}

.lesson-box h4 { color: var(--green-text); margin: 0 0 6px; font-size: 0.9rem; }
.lesson-box p { color: var(--text-secondary); font-size: 0.9rem; margin: 0; }

.action-box {
    background: var(--blue-bg);
    border: 1px solid var(--blue-border);
    border-radius: 6px;
    padding: 20px 24px;
    margin: 28px 0;
    text-align: center;
}

.action-box h3 { color: var(--accent); margin-bottom: 12px; font-size: 1.05rem; }

.btn {
    display: inline-block;
    padding: 8px 20px;
    border-radius: 4px;
    font-size: 0.9rem;
    font-weight: 500;
    text-decoration: none;
    cursor: pointer;
    border: 1px solid transparent;
    margin: 4px;
    transition: background 0.2s, border-color 0.2s;
}

.btn-primary {
    background: var(--accent);
    color: white;
    border-color: var(--accent);
}
.btn-primary:hover { background: var(--accent-hover); color: white; text-decoration: none; }

.btn-outline {
    background: var(--bg);
    color: var(--text-secondary);
    border-color: var(--border-dark);
}
.btn-outline:hover { border-color: var(--text-muted); color: var(--text); text-decoration: none; }

/* Command blocks */
.cmd-block {
    background: var(--bg-code);
    padding: 12px 16px;
    border-radius: 6px;
    margin: 8px 0;
    font-family: var(--font-mono);
    font-size: 0.875rem;
    color: var(--code-text);
    border: 1px solid var(--border);
}

.cmd-desc {
    font-size: 0.85rem;
    color: var(--text-muted);
    margin-top: 4px;
}

/* Responsive */
@media (max-width: 900px) {
    .sidebar {
        transform: translateX(-100%);
        transition: transform 0.2s;
    }
    .sidebar.open {
        transform: translateX(0);
        box-shadow: 4px 0 20px rgba(0,0,0,0.3);
    }
    .topbar {
        display: flex;
    }
    .content {
        margin-left: 0;
        padding: 72px 20px 40px;
    }
}

@media print {
    .sidebar, .topbar { display: none; }
    .content { margin-left: 0; padding: 20px; }
    .theme-toggle, .copy-btn { display: none; }
}