:root {
  --bg: #F2F5F4; --bg-raised: #FAFCFB; --ink: #0E1E24; --ink-soft: #44555B; --ink-faint: #74858A;
  --line: #D3DDDC; --signal: #0E86A8; --accent: #4FC6A6;
  --serif: "Instrument Serif", Georgia, serif;
  --sans: "Geist", system-ui, -apple-system, sans-serif;
  --mono: "Geist Mono", ui-monospace, Menlo, monospace;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #061218; --bg-raised: #0B1B22; --ink: #E6F0F0; --ink-soft: #A3B6B8; --ink-faint: #6A7E82;
    --line: #1A3038; --signal: #3DBAD6;
  }
}
:root[data-theme="dark"] {
  --bg: #061218; --bg-raised: #0B1B22; --ink: #E6F0F0; --ink-soft: #A3B6B8; --ink-faint: #6A7E82;
  --line: #1A3038; --signal: #3DBAD6;
}
*, *::before, *::after { box-sizing: border-box; }
html { scroll-behavior: smooth; -webkit-text-size-adjust: 100%; }
body { margin: 0; background: var(--bg); color: var(--ink); font: 400 16px/1.7 var(--sans); -webkit-font-smoothing: antialiased; transition: background-color .4s, color .4s; }
a { color: var(--signal); text-underline-offset: 3px; }
.wrap { max-width: 1080px; margin: 0 auto; padding: 0 16px; }
@media (min-width: 720px) { .wrap { padding: 0 32px; } }

header { position: sticky; top: 0; z-index: 10; background: color-mix(in srgb, var(--bg) 85%, transparent); backdrop-filter: blur(10px); -webkit-backdrop-filter: blur(10px); border-bottom: 1px solid var(--line); }
.bar { display: flex; align-items: center; justify-content: space-between; height: 64px; gap: 12px; }
.logo { display: inline-flex; align-items: center; gap: 10px; color: var(--ink); text-decoration: none; font-weight: 600; font-size: 18px; }
.logo svg { width: 32px; height: 32px; }
.bar-right { display: flex; gap: 8px; align-items: center; }
.back { font-size: 14px; color: var(--ink-soft); text-decoration: none; border: 1px solid var(--line); border-radius: 999px; padding: 7px 14px; white-space: nowrap; }
.back:hover { color: var(--ink); border-color: var(--ink-faint); }
.icon-btn { width: 36px; height: 36px; border-radius: 999px; border: 1px solid var(--line); background: transparent; color: var(--ink-soft); cursor: pointer; display: grid; place-items: center; }
.icon-btn:hover { color: var(--ink); }
.icon-btn .i-sun { display: none; }
:root[data-theme="dark"] .icon-btn .i-sun { display: block; }
:root[data-theme="dark"] .icon-btn .i-moon { display: none; }

.hero { padding: 64px 0 40px; border-bottom: 1px solid var(--line); }
.eyebrow { font-family: var(--mono); font-size: 12px; letter-spacing: .08em; text-transform: uppercase; color: var(--ink-faint); }
h1 { font-family: var(--serif); font-weight: 400; font-size: clamp(44px, 7vw, 76px); line-height: 1; letter-spacing: -.02em; margin: 14px 0 16px; }
.meta { color: var(--ink-faint); font-family: var(--mono); font-size: 13px; }
.lede { color: var(--ink-soft); font-size: 18px; max-width: 62ch; margin: 20px 0 0; }

.layout { display: grid; gap: 40px; padding: 48px 0 96px; }
@media (min-width: 900px) { .layout { grid-template-columns: 240px 1fr; gap: 64px; } }
.toc { font-size: 14px; }
@media (min-width: 900px) { .toc { position: sticky; top: 96px; align-self: start; } }
.toc b { display: block; font-family: var(--mono); font-size: 12px; font-weight: 400; text-transform: uppercase; letter-spacing: .08em; color: var(--ink-faint); margin-bottom: 12px; }
.toc ol { list-style: none; margin: 0; padding: 0; display: grid; gap: 2px; counter-reset: t; }
.toc li { counter-increment: t; }
.toc a { display: flex; gap: 10px; padding: 6px 10px; border-radius: 8px; color: var(--ink-soft); text-decoration: none; }
.toc a::before { content: counter(t, decimal-leading-zero); font-family: var(--mono); font-size: 12px; color: var(--ink-faint); padding-top: 2px; }
.toc a:hover, .toc a.active { background: var(--bg-raised); color: var(--ink); }

article { min-width: 0; max-width: 72ch; }
article section { padding-top: 8px; margin-bottom: 40px; scroll-margin-top: 88px; }
article h2 { font-family: var(--serif); font-weight: 400; font-size: 32px; line-height: 1.15; margin: 0 0 12px; }
article h3 { font-size: 17px; font-weight: 600; margin: 20px 0 6px; }
article p, article li { color: var(--ink-soft); }
article ul { padding-left: 20px; }
article li + li { margin-top: 6px; }
article strong { color: var(--ink); font-weight: 500; }
.note { background: var(--bg-raised); border: 1px solid var(--line); border-left: 3px solid var(--signal); border-radius: 10px; padding: 14px 18px; margin: 16px 0; }
.note p { margin: 0; }
mark.ph { background: color-mix(in srgb, var(--accent) 45%, transparent); color: var(--ink); padding: 0 4px; border-radius: 4px; }

footer { border-top: 1px solid var(--line); padding: 32px 0 40px; color: var(--ink-faint); font-size: 14px; }
.foot { display: flex; flex-wrap: wrap; justify-content: space-between; gap: 16px; }
.foot nav { display: flex; flex-wrap: wrap; gap: 20px; }
.foot a { color: inherit; text-decoration: none; }
.foot a:hover { color: var(--ink); }
:focus-visible { outline: 2px solid var(--signal); outline-offset: 3px; border-radius: 6px; }

/* Language switch */
html[data-lang="vi"] .l-en, html[data-lang="en"] .l-vi { display: none !important; }
.lang { display: inline-flex; border: 1px solid var(--line); border-radius: 999px; padding: 2px; }
.lang button { border: 0; background: transparent; color: var(--ink-faint); font: 500 12px/1 var(--mono); padding: 8px 10px; border-radius: 999px; cursor: pointer; }
html[data-lang="vi"] [data-set-lang="vi"], html[data-lang="en"] [data-set-lang="en"] { background: var(--signal); color: var(--bg); }
.kv { list-style: none; padding: 0; margin: 12px 0 0; border-top: 1px solid var(--line); }
.kv li { display: grid; grid-template-columns: 150px 1fr; gap: 12px; padding: 10px 0; border-bottom: 1px solid var(--line); }
.kv li span { font-family: var(--mono); font-size: 12px; text-transform: uppercase; letter-spacing: .05em; color: var(--ink-faint); padding-top: 3px; }
@media (max-width: 520px) { .kv li { grid-template-columns: 1fr; gap: 2px; } .back { display: none; } }
.foot-co { display: grid; gap: 4px; }
.logo .logo-mark { width: 32px; height: 32px; }