:root {
    --bg: #ffffff;
    --bg-soft: #f6f7f9;
    --fg: #1b1f24;
    --fg-muted: #5b6470;
    --accent: #1a56a0;
    --accent-soft: #e8f0fc;
    --border: #d8dee6;
    --code-bg: #f3f4f6;
    --sidebar-width: 20rem;
    --content-width: 46rem;
}

@media (prefers-color-scheme: dark) {
    :root {
        --bg: #12151a;
        --bg-soft: #181c22;
        --fg: #e6e9ee;
        --fg-muted: #9aa4b2;
        --accent: #7ea6ff;
        --accent-soft: #1d2637;
        --border: #2a313b;
        --code-bg: #1b2029;
    }
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }

body {
    margin: 0;
    background: var(--bg);
    color: var(--fg);
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
    font-size: 17px;
    line-height: 1.65;
}

/* ── Layout ── */
.layout {
    display: grid;
    grid-template-columns: var(--sidebar-width) minmax(0, 1fr);
    min-height: 100vh;
}

/* ── Sidebar ── */
.sidebar {
    position: sticky;
    top: 0;
    height: 100vh;
    overflow-y: auto;
    background: var(--bg-soft);
    border-right: 1px solid var(--border);
    padding: 1.5rem 1rem;
    display: flex;
    flex-direction: column;
}

.brand {
    font-size: 1.15rem;
    font-weight: 700;
    color: var(--accent);
    text-decoration: none;
    display: block;
}

.brand-sub {
    font-size: 0.8rem;
    color: var(--fg-muted);
    display: block;
    margin-bottom: 1.2rem;
}

.sidebar ol {
    list-style: none;
    margin: 0;
    padding: 0;
    flex: 1;
}

.sidebar ol li a {
    display: block;
    padding: 0.25rem 0.5rem;
    border-radius: 4px;
    color: var(--fg-muted);
    text-decoration: none;
    font-size: 0.85rem;
    line-height: 1.4;
}

.sidebar ol li a:hover,
.sidebar ol li a[aria-current="page"] {
    background: var(--accent-soft);
    color: var(--accent);
}

.part-label {
    font-size: 0.72rem;
    font-weight: 700;
    text-transform: uppercase;
    letter-spacing: 0.07em;
    color: var(--fg-muted);
    padding: 1rem 0.5rem 0.3rem;
}

.download {
    display: block;
    margin-top: 1.2rem;
    padding: 0.5rem 0.75rem;
    border: 1px solid var(--border);
    border-radius: 5px;
    text-align: center;
    font-size: 0.82rem;
    color: var(--accent);
    text-decoration: none;
}

.download:hover { background: var(--accent-soft); }

/* ── Main content ── */
main {
    max-width: var(--content-width);
    padding: 2.5rem 2rem 5rem;
}

h1.title {
    color: var(--accent);
    border-bottom: 2px solid var(--border);
    padding-bottom: 0.4rem;
    margin-top: 0;
    font-size: 1.9rem;
}

h1, h2, h3, h4 { line-height: 1.25; }

h1 {
    color: var(--accent);
    border-bottom: 1px solid var(--border);
    padding-bottom: 0.3rem;
    margin-top: 2.5rem;
    font-size: 1.6rem;
}

h2 { margin-top: 1.8rem; font-size: 1.25rem; }
h3 { margin-top: 1.4rem; font-size: 1.05rem; }

/* ── Landing page ── */
.landing-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(16rem, 1fr));
    gap: 0.75rem;
    margin: 1rem 0 2rem;
}

.landing-grid a {
    display: block;
    padding: 0.9rem 1rem;
    border: 1px solid var(--border);
    border-radius: 6px;
    text-decoration: none;
    color: var(--fg);
    font-size: 0.9rem;
    line-height: 1.4;
}

.landing-grid a:hover { border-color: var(--accent); background: var(--accent-soft); }

.landing-grid .num {
    display: block;
    font-size: 0.72rem;
    color: var(--fg-muted);
    margin-bottom: 0.2rem;
}

.build-meta {
    font-size: 0.8rem;
    color: var(--fg-muted);
    margin-top: 3rem;
}

/* ── Prev/next pager ── */
.pager {
    display: flex;
    justify-content: space-between;
    margin-top: 3rem;
    padding-top: 1rem;
    border-top: 1px solid var(--border);
    font-size: 0.9rem;
}

.pager a { color: var(--accent); text-decoration: none; }
.pager a:hover { text-decoration: underline; }
.spacer { flex: 1; }

/* ── Links ── */
a { color: var(--accent); }

/* ── Code ── */
code, pre {
    font-family: "SFMono-Regular", Consolas, "Liberation Mono", monospace;
    font-size: 0.84em;
}

pre {
    background: var(--code-bg);
    border: 1px solid var(--border);
    border-radius: 6px;
    padding: 0.9rem 1.1rem;
    overflow-x: auto;
    line-height: 1.45;
}

code { background: var(--code-bg); border-radius: 3px; padding: 0.1em 0.3em; }
pre code { background: none; padding: 0; }

/* ── Tables ── */
table { border-collapse: collapse; width: 100%; margin: 1.2rem 0; font-size: 0.92em; }
th { background: var(--bg-soft); font-weight: 600; }
th, td { border: 1px solid var(--border); padding: 0.45rem 0.7rem; text-align: left; vertical-align: top; }
tr:nth-child(even) td { background: var(--bg-soft); }

/* ── Images / diagrams ── */
img, svg { max-width: 100%; height: auto; display: block; margin: 1.2rem auto; }

/* ── Math ── */
.math.display { overflow-x: auto; }

/* ── Blockquotes ── */
blockquote {
    border-left: 3px solid var(--accent);
    margin: 1rem 0;
    padding: 0.4rem 1rem;
    color: var(--fg-muted);
    background: var(--accent-soft);
    border-radius: 0 4px 4px 0;
}

hr { border: none; border-top: 1px solid var(--border); margin: 2rem 0; }

/* ── Responsive: collapse sidebar on small screens ── */
@media (max-width: 860px) {
    .layout { grid-template-columns: 1fr; }
    .sidebar { position: static; height: auto; border-right: none; border-bottom: 1px solid var(--border); }
    main { padding: 1.5rem 1rem 4rem; }
}