:root {
    --bg: #0d1117;
    --surface: #161b22;
    --border: #30363d;
    --text: #c9d1d9;
    --muted: #8b949e;
    --accent: #58a6ff;
    --green: #3fb950;
    --yellow: #d29922;
    --red: #f85149;
    --code-bg: #1f2937;
    --tag-bg: #1c2d3f;
}

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

body {
    background: var(--bg);
    color: var(--text);
    font-family: -apple-system, 'Segoe UI', sans-serif;
    font-size: 15px;
    line-height: 1.7;
}

/* ── Layout ── */
.page {
    max-width: 860px;
    margin: 0 auto;
    padding: 48px 24px 80px;
}

/* ── Header ── */
.doc-header {
    border-bottom: 1px solid var(--border);
    padding-bottom: 24px;
    margin-bottom: 48px;
}

.doc-header h1 {
    font-size: 2rem;
    color: #fff;
    letter-spacing: -.5px;
}

.doc-header .subtitle {
    color: var(--muted);
    margin-top: 6px;
    font-size: 14px;
}

.tag {
    display: inline-block;
    background: var(--tag-bg);
    color: var(--accent);
    border: 1px solid #1f4068;
    border-radius: 20px;
    font-size: 12px;
    padding: 2px 10px;
    margin-right: 6px;
}

/* ── Sections ── */
section {
    margin-bottom: 52px;
}

.section-label {
    font-size: 11px;
    font-weight: 700;
    letter-spacing: 1.5px;
    text-transform: uppercase;
    color: var(--accent);
    margin-bottom: 10px;
}

h2 {
    font-size: 1.3rem;
    color: #fff;
    margin-bottom: 16px;
    padding-bottom: 8px;
    border-bottom: 1px solid var(--border);
}

p {
    margin-bottom: 12px;
    color: var(--text);
}

/* ── Problem table ── */
.problem-grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 12px;
    margin-top: 4px;
}

.problem-card {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: 8px;
    padding: 16px 18px;
}

.problem-card .fmt {
    font-family: 'Cascadia Code', 'Fira Code', monospace;
    font-size: 1rem;
    color: var(--yellow);
    margin-bottom: 8px;
}

.problem-card p {
    font-size: 13px;
    color: var(--muted);
    margin: 0;
}

/* ── Goals ── */
.goal-list {
    list-style: none;
    display: flex;
    flex-direction: column;
    gap: 10px;
}

.goal-list li {
    display: flex;
    align-items: flex-start;
    gap: 12px;
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: 8px;
    padding: 14px 16px;
}

.goal-icon {
    font-size: 18px;
    line-height: 1;
    flex-shrink: 0;
}

.goal-list strong {
    color: #fff;
    display: block;
    margin-bottom: 2px;
}

.goal-list span {
    font-size: 13px;
    color: var(--muted);
}

/* ── Design sub-sections ── */
.design-block {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: 8px;
    padding: 20px 22px;
    margin-bottom: 14px;
}

.design-block h3 {
    font-size: 14px;
    color: var(--accent);
    margin-bottom: 10px;
}

.design-block p,
.design-block li {
    font-size: 14px;
}

.design-block ul {
    padding-left: 18px;
    color: var(--muted);
}

.design-block li {
    margin-bottom: 4px;
}

/* ── Code ── */
pre {
    background: var(--code-bg);
    border: 1px solid var(--border);
    border-radius: 8px;
    padding: 16px 18px;
    overflow-x: auto;
    font-family: 'Cascadia Code', 'Fira Code', 'Consolas', monospace;
    font-size: 13px;
    line-height: 1.6;
}

code {
    font-family: inherit;
}

.kw {
    color: #ff7b72;
}

/* keyword  */
.cm {
    color: #6e7681;
}

/* comment  */
.st {
    color: #a5d6ff;
}

/* string   */
.nm {
    color: #79c0ff;
}

/* name/var */
.nb {
    color: #d2a8ff;
}

/* builtin  */
.op {
    color: #ffa657;
}

/* operator */

/* ── File structure ── */
.struct-wrap {
    display: grid;
    grid-template-columns: 1fr 1fr;
    gap: 12px;
}

.struct-panel {
    background: var(--code-bg);
    border: 1px solid var(--border);
    border-radius: 8px;
    overflow: hidden;
}

.struct-panel .panel-title {
    background: #2d333b;
    padding: 8px 14px;
    font-size: 12px;
    font-weight: 600;
    color: var(--muted);
    letter-spacing: .5px;
    border-bottom: 1px solid var(--border);
}

.struct-panel pre {
    border: none;
    border-radius: 0;
    margin: 0;
    font-size: 12.5px;
}

.field-req {
    color: var(--green);
}

.field-opt {
    color: var(--muted);
}

.field-type {
    color: #d2a8ff;
}

/* ── Note callout ── */
.callout {
    display: flex;
    gap: 12px;
    background: #1c2d1c;
    border: 1px solid #2ea04326;
    border-left: 3px solid var(--green);
    border-radius: 6px;
    padding: 12px 16px;
    margin-top: 14px;
    font-size: 13.5px;
    color: #adbcad;
}

.callout-icon {
    flex-shrink: 0;
    font-size: 16px;
}

/* ── Result ── */
.insight {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: 8px;
    padding: 20px 22px;
}

.insight p {
    margin-bottom: 8px;
    font-size: 14px;
}

.insight p:last-child {
    margin-bottom: 0;
}

.insight strong {
    color: #fff;
}

/* ── Code examples ── */
.lang-tabs {
    display: flex;
    gap: 2px;
    margin-bottom: -1px;
}

.lang-tab {
    padding: 6px 16px;
    font-size: 13px;
    border-radius: 6px 6px 0 0;
    background: var(--border);
    color: var(--muted);
    cursor: pointer;
    border: 1px solid var(--border);
    border-bottom: none;
    -webkit-user-select: none;
    user-select: none;
}

.lang-tab.active {
    background: var(--code-bg);
    color: var(--accent);
    border-color: var(--border);
}

.code-panel {
    display: none;
}

.code-panel.active {
    display: block;
}

.code-panel pre {
    border-radius: 0 8px 8px 8px;
}

@media (max-width: 600px) {
    .problem-grid {
        grid-template-columns: 1fr;
    }

    .struct-wrap {
        grid-template-columns: 1fr;
    }
}