/* ==========================================================================
   System Design Guide — styles
   Light by default, dark when the device asks for it, or when toggled (data-theme).
   ========================================================================== */

:root {
  --bg: #faf8f3;
  --bg-soft: #f2eee4;
  --panel: #ffffff;
  --ink: #1c2421;
  --ink-soft: #4b5652;
  --ink-faint: #7a847f;
  --line: #e2dccd;
  --accent: #1d6f86;
  --accent-soft: #e2f0f3;
  --mark: #f7d30e;
  --good: #2e8a5a;
  --bad: #c2362f;
  --warn: #b7791f;

  /* diagram kinds */
  --k-client: #3b6fd8;
  --k-lb: #1d6f86;
  --k-service: #4b5652;
  --k-db: #b8901a;
  --k-cache: #d0602c;
  --k-queue: #8a5fb0;
  --k-cdn: #2e8a5a;
  --k-store: #8c6a3c;
  --k-worker: #6a7a2e;
  --k-ext: #8a8f8c;

  --sans: ui-sans-serif, system-ui, -apple-system, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  --mono: ui-monospace, "SF Mono", Menlo, Consolas, monospace;
  --top: 60px;
  color-scheme: light;
}

@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #151b19; --bg-soft: #1c2422; --panel: #1f2826; --ink: #ece4d3; --ink-soft: #b9b2a3; --ink-faint: #868b84;
    --line: #2f3a37; --accent: #5fb4cf; --accent-soft: #1b3038; --mark: #f7d30e; --good: #5cc28a; --bad: #f0736b; --warn: #e3b25a;
    --k-client: #7ea4f0; --k-lb: #5fb4cf; --k-service: #b9b2a3; --k-db: #e3c45a; --k-cache: #f08a5a; --k-queue: #b896dc;
    --k-cdn: #5cc28a; --k-store: #cfa874; --k-worker: #b4c46a; --k-ext: #868b84;
    color-scheme: dark;
  }
}
:root[data-theme="dark"] {
  --bg: #151b19; --bg-soft: #1c2422; --panel: #1f2826; --ink: #ece4d3; --ink-soft: #b9b2a3; --ink-faint: #868b84;
  --line: #2f3a37; --accent: #5fb4cf; --accent-soft: #1b3038; --mark: #f7d30e; --good: #5cc28a; --bad: #f0736b; --warn: #e3b25a;
  --k-client: #7ea4f0; --k-lb: #5fb4cf; --k-service: #b9b2a3; --k-db: #e3c45a; --k-cache: #f08a5a; --k-queue: #b896dc;
  --k-cdn: #5cc28a; --k-store: #cfa874; --k-worker: #b4c46a; --k-ext: #868b84;
  color-scheme: dark;
}

/* --- Base ------------------------------------------------------------------ */
*, *::before, *::after { box-sizing: border-box; }
html { scroll-padding-top: calc(var(--top) + 16px); -webkit-text-size-adjust: 100%; }
body {
  margin: 0;
  background: var(--bg);
  color: var(--ink);
  font: 17px/1.65 var(--sans);
  -webkit-font-smoothing: antialiased;
}
a { color: var(--accent); text-underline-offset: 3px; }
a:hover { text-decoration-thickness: 2px; }
img, svg { max-width: 100%; }
:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; }
::selection { background: var(--mark); color: #1c2421; }
code, kbd { font: 0.88em/1 var(--mono); background: var(--bg-soft); border: 1px solid var(--line); border-radius: 4px; padding: 2px 5px; }
pre { overflow-x: auto; background: var(--bg-soft); border: 1px solid var(--line); border-radius: 8px; padding: 14px 16px; font: 14px/1.55 var(--mono); }
pre code { background: none; border: 0; padding: 0; font-size: inherit; }

.skip { position: absolute; left: -9999px; top: 8px; z-index: 100; background: var(--mark); color: #1c2421; padding: 8px 14px; }
.skip:focus { left: 8px; }

/* --- Top bar ----------------------------------------------------------------- */
.top {
  position: sticky;
  top: 0;
  z-index: 40;
  height: var(--top);
  display: flex;
  align-items: center;
  gap: 16px;
  padding: 0 20px;
  background: color-mix(in srgb, var(--bg) 92%, transparent);
  backdrop-filter: blur(8px);
  border-bottom: 1px solid var(--line);
}
.brand { display: flex; align-items: center; gap: 10px; color: var(--ink); text-decoration: none; font-weight: 600; white-space: nowrap; }
.brand img { image-rendering: pixelated; }
.brand b { color: var(--accent); }
.top__menu, .top__theme {
  display: grid;
  place-items: center;
  width: 38px;
  height: 38px;
  padding: 0;
  background: none;
  border: 1px solid var(--line);
  border-radius: 8px;
  color: var(--ink);
  cursor: pointer;
}
.top__menu { display: none; }
.top__menu svg, .top__theme svg { width: 20px; height: 20px; fill: none; stroke: currentColor; stroke-width: 2; stroke-linecap: round; }
.top__theme { margin-left: auto; }

.search { position: relative; flex: 1; max-width: 440px; margin-left: 24px; }
.search__input {
  width: 100%;
  height: 38px;
  padding: 0 12px;
  font: inherit;
  font-size: 15px;
  color: var(--ink);
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: 8px;
}
.search__results {
  position: absolute;
  top: 44px;
  left: 0;
  right: 0;
  max-height: 60vh;
  overflow: auto;
  margin: 0;
  padding: 6px;
  list-style: none;
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: 10px;
  box-shadow: 0 12px 30px rgba(0, 0, 0, 0.18);
}
.search__results a { display: block; padding: 8px 10px; border-radius: 6px; color: var(--ink); text-decoration: none; }
.search__results a:hover, .search__results a.is-active { background: var(--accent-soft); }
.search__results small { display: block; color: var(--ink-faint); font-size: 13px; }
.search__results .search__empty { padding: 10px; color: var(--ink-faint); }

/* --- Layout ------------------------------------------------------------------ */
.layout { display: grid; grid-template-columns: 280px minmax(0, 1fr); }
.sidebar {
  position: sticky;
  top: var(--top);
  height: calc(100vh - var(--top));
  overflow-y: auto;
  padding: 22px 16px 40px 20px;
  border-right: 1px solid var(--line);
  font-size: 15px;
}
.side__head {
  display: flex;
  justify-content: space-between;
  margin: 24px 0 6px;
  padding-left: 8px;
  font-size: 12px;
  font-weight: 700;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--ink-faint);
}
.side__count { font-weight: 600; letter-spacing: 0; color: var(--good); }
.side__list { margin: 0; padding: 0; list-style: none; }
.side__link {
  position: relative;
  display: flex;
  gap: 8px;
  padding: 6px 28px 6px 8px;
  border-radius: 6px;
  color: var(--ink-soft);
  text-decoration: none;
  line-height: 1.35;
}
.side__link:hover { background: var(--bg-soft); color: var(--ink); }
.side__link.is-current { background: var(--accent-soft); color: var(--ink); font-weight: 600; }
.side__num { flex: none; width: 24px; font: 12px/1.6 var(--mono); color: var(--ink-faint); }
.side__link.is-done::after { content: "✓"; position: absolute; right: 8px; color: var(--good); font-weight: 700; }

.main { min-width: 0; padding: 40px clamp(20px, 4vw, 56px) 80px; }

/* --- Docs ---------------------------------------------------------------------- */
.doc { display: grid; grid-template-columns: minmax(0, 760px) 220px; gap: 56px; }
.doc__body { min-width: 0; }
.eyebrow { margin: 0 0 8px; font: 600 13px/1.4 var(--mono); letter-spacing: 0.06em; text-transform: uppercase; color: var(--accent); }
h1, h2, h3 { line-height: 1.2; letter-spacing: -0.01em; }
h1 { margin: 0; font-size: clamp(2rem, 4vw, 2.7rem); }
.lede { margin: 14px 0 0; font-size: 1.18rem; color: var(--ink-soft); }
.doc__meta { margin: 12px 0 0; font-size: 14px; color: var(--ink-faint); }

.prose h2 { margin: 2.6em 0 0.6em; padding-top: 0.6em; font-size: 1.6rem; border-top: 1px solid var(--line); }
.prose h3 { margin: 1.8em 0 0.4em; font-size: 1.2rem; }
.prose p, .prose ul, .prose ol { margin: 0.8em 0; }
.prose li { margin: 0.3em 0; }
.prose li > p { margin: 0.2em 0; }
.prose strong { color: var(--ink); }
.prose blockquote { margin: 1.2em 0; padding: 4px 18px; border-left: 4px solid var(--mark); color: var(--ink-soft); font-style: italic; }

.table-scroll { margin: 1.2em 0; overflow-x: auto; border: 1px solid var(--line); border-radius: 10px; }
table { width: 100%; border-collapse: collapse; font-size: 15px; }
th, td { padding: 10px 14px; text-align: left; vertical-align: top; border-bottom: 1px solid var(--line); }
th { background: var(--bg-soft); font-size: 13px; letter-spacing: 0.02em; }
tr:last-child td { border-bottom: 0; }

.glance { margin: 28px 0 8px; padding: 16px 20px; background: var(--accent-soft); border-radius: 12px; }
.glance__title { margin: 0 0 4px; font: 700 12px/1.4 var(--mono); letter-spacing: 0.08em; text-transform: uppercase; color: var(--accent); }
.glance ul { margin: 0; padding-left: 20px; }
.glance li { margin: 4px 0; }

/* callouts: tip, warn, interview, think */
.callout { margin: 1.4em 0; padding: 14px 18px; background: var(--panel); border: 1px solid var(--line); border-left: 4px solid var(--accent); border-radius: 8px; }
.callout > :last-child { margin-bottom: 0; }
.callout p { margin: 0.5em 0; }
.callout__title { margin: 0 0 4px !important; font: 700 13px/1.4 var(--mono); letter-spacing: 0.04em; text-transform: uppercase; color: var(--accent); }
.callout--warn { border-left-color: var(--warn); }
.callout--warn .callout__title { color: var(--warn); }
.callout--interview { border-left-color: var(--mark); }
.callout--interview .callout__title { color: var(--ink); }
.callout--think { border-left-color: var(--k-queue); font-style: italic; }
.callout--think .callout__title { color: var(--k-queue); font-style: normal; }

.procon { display: grid; grid-template-columns: 1fr 1fr; gap: 14px; margin: 1.2em 0; }
.procon__col { padding: 12px 16px; background: var(--panel); border: 1px solid var(--line); border-radius: 10px; border-top: 4px solid var(--good); }
.procon__col--con { border-top-color: var(--bad); }
.procon__title { margin: 0 0 4px !important; font: 700 13px/1.4 var(--mono); text-transform: uppercase; letter-spacing: 0.06em; color: var(--good); }
.procon__col--con .procon__title { color: var(--bad); }
.procon--compare .procon__col { border-top-color: var(--accent); }
.procon--compare .procon__col--con { border-top-color: var(--k-queue); }
.procon--compare .procon__title { color: var(--accent); }
.procon--compare .procon__col--con .procon__title { color: var(--k-queue); }
.procon ul { margin: 0; padding-left: 18px; }
.procon li { margin: 5px 0; font-size: 15.5px; }

/* --- Diagrams (rendered by the {% diagram %} shortcode) --------------------------- */
.diagram { margin: 1.6em 0; }
.diagram__scroll { overflow-x: auto; padding: 8px; background: var(--panel); border: 1px solid var(--line); border-radius: 12px; }
.diagram svg.d { display: block; height: auto; margin: 0 auto; min-width: 560px; }   /* phones scroll, not shrink */
.diagram figcaption { margin-top: 8px; font-size: 14px; color: var(--ink-faint); text-align: center; }
.d { font: 14px/1 var(--sans); }
.d-node { --k: var(--k-service); }
.d-node--client { --k: var(--k-client); }
.d-node--lb { --k: var(--k-lb); }
.d-node--db { --k: var(--k-db); }
.d-node--cache { --k: var(--k-cache); }
.d-node--queue { --k: var(--k-queue); }
.d-node--cdn { --k: var(--k-cdn); }
.d-node--store { --k: var(--k-store); }
.d-node--worker { --k: var(--k-worker); }
.d-node--ext { --k: var(--k-ext); }
.d-shape { fill: color-mix(in srgb, var(--k) 12%, var(--panel)); stroke: var(--k); stroke-width: 1.8; }
.d-shape--rim { fill: none; }
.d-node--ext .d-shape { stroke-dasharray: 5 4; }
.d-label { fill: var(--ink); font-weight: 600; text-anchor: middle; }
.d-edge { fill: none; stroke: var(--ink-faint); stroke-width: 1.8; }
.d-edge--async { stroke-dasharray: 6 5; }
.d-head { fill: var(--ink-faint); }
.d-elabel { fill: var(--ink-soft); font-size: 12.5px; paint-order: stroke; stroke: var(--panel); stroke-width: 5px; stroke-linejoin: round; }
.d-group { fill: color-mix(in srgb, var(--accent) 5%, transparent); stroke: var(--line); stroke-width: 1.5; stroke-dasharray: 7 5; }
.d-glabel { fill: var(--ink-faint); font: 600 12px/1 var(--mono); letter-spacing: 0.04em; text-transform: uppercase; }

/* --- Self-test cards and flashcards ---------------------------------------------- */
.selftest { margin-top: 3em; padding-top: 1em; border-top: 1px solid var(--line); }
.selftest h2 { font-size: 1.6rem; margin: 0.4em 0 0.2em; }
.selftest__hint { color: var(--ink-faint); font-size: 15px; }
.cards { display: grid; grid-template-columns: repeat(auto-fill, minmax(230px, 1fr)); gap: 12px; margin-top: 14px; }
.card {
  min-height: 120px;
  padding: 16px;
  text-align: left;
  font: inherit;
  font-size: 15.5px;
  line-height: 1.45;
  color: var(--ink);
  background: var(--panel);
  border: 1px solid var(--line);
  border-bottom-width: 3px;
  border-radius: 10px;
  cursor: pointer;
}
.card:hover { border-color: var(--accent); }
.card__front { display: block; font-weight: 600; }
.card__back { display: none; margin-top: 10px; padding-top: 10px; border-top: 1px dashed var(--line); color: var(--ink-soft); }
.card[aria-expanded="true"] { border-bottom-color: var(--good); }
.card[aria-expanded="true"] .card__back { display: block; }

.doc__done { margin-top: 2.4em; }
.done-btn {
  padding: 10px 18px;
  font: 600 15px/1 var(--sans);
  color: var(--ink);
  background: var(--panel);
  border: 1.5px solid var(--line);
  border-radius: 999px;
  cursor: pointer;
}
.done-btn[aria-pressed="true"] { color: #fff; background: var(--good); border-color: var(--good); }
.done-btn[aria-pressed="true"]::before { content: "✓ "; }

.pager { display: grid; grid-template-columns: 1fr 1fr; gap: 14px; margin-top: 28px; }
.pager a { display: block; padding: 14px 16px; border: 1px solid var(--line); border-radius: 10px; color: var(--ink); text-decoration: none; font-weight: 600; }
.pager a:hover { border-color: var(--accent); }
.pager span { display: block; margin-bottom: 2px; font-size: 13px; font-weight: 400; color: var(--ink-faint); }
.pager__next { text-align: right; grid-column: 2; }

.toc { position: sticky; top: calc(var(--top) + 32px); align-self: start; max-height: calc(100vh - var(--top) - 64px); overflow-y: auto; font-size: 14px; }
.toc__title { margin: 0 0 8px; font: 700 12px/1.4 var(--mono); letter-spacing: 0.08em; text-transform: uppercase; color: var(--ink-faint); }
.toc ol { margin: 0; padding: 0; list-style: none; border-left: 2px solid var(--line); }
.toc a { display: block; padding: 4px 0 4px 12px; margin-left: -2px; border-left: 2px solid transparent; color: var(--ink-soft); text-decoration: none; }
.toc a:hover { color: var(--ink); }
.toc a.is-active { color: var(--accent); border-left-color: var(--accent); }

/* --- Home ------------------------------------------------------------------------- */
.home { max-width: 1040px; }
.hero { padding: 20px 0 10px; }
.hero h1 { font-size: clamp(2.3rem, 5vw, 3.4rem); max-width: 16ch; }
.hero .lede { max-width: 60ch; }
.hero__cta { display: flex; flex-wrap: wrap; gap: 12px; margin-top: 24px; }
.btn { display: inline-block; padding: 12px 20px; border-radius: 999px; font-weight: 600; text-decoration: none; background: var(--accent); color: #fff; border: 1.5px solid var(--accent); }
.btn:hover { filter: brightness(1.08); }
.btn--ghost { background: none; color: var(--accent); }
.stats { display: flex; flex-wrap: wrap; gap: 28px; margin-top: 28px; color: var(--ink-soft); font-size: 15px; }
.stats b { display: block; font-size: 1.6rem; color: var(--ink); }
.home h2 { margin: 2.4em 0 0.4em; font-size: 1.7rem; }
.home .section-lede { margin: 0 0 1em; color: var(--ink-soft); max-width: 65ch; }
.grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(240px, 1fr)); gap: 14px; }
.tile { position: relative; display: block; padding: 16px 18px; background: var(--panel); border: 1px solid var(--line); border-radius: 12px; color: var(--ink); text-decoration: none; }
.tile:hover { border-color: var(--accent); }
.tile__num { font: 12px/1 var(--mono); color: var(--ink-faint); }
.tile h3 { margin: 6px 0 4px; font-size: 1.05rem; }
.tile p { margin: 0; font-size: 14.5px; line-height: 1.5; color: var(--ink-soft); }
.tile.is-done::after { content: "✓ done"; position: absolute; top: 14px; right: 16px; font: 700 12px/1 var(--mono); color: var(--good); }
.plan { counter-reset: step; margin: 0; padding: 0; list-style: none; display: grid; gap: 10px; }
.plan li { position: relative; padding: 12px 16px 12px 56px; background: var(--panel); border: 1px solid var(--line); border-radius: 10px; }
.plan li::before { counter-increment: step; content: counter(step); position: absolute; left: 16px; top: 12px; width: 26px; height: 26px; display: grid; place-items: center; border-radius: 50%; background: var(--accent); color: #fff; font: 700 13px/1 var(--mono); }

/* --- Flashcards page ---------------------------------------------------------------- */
.fc-actions { display: flex; flex-wrap: wrap; align-items: center; gap: 12px; margin: 22px 0; }
.fc-select { max-width: 100%; padding: 11px 14px; font: 15px/1.2 var(--sans); color: var(--ink); background: var(--panel); border: 1.5px solid var(--line); border-radius: 999px; }
.fc-topic h2 a { color: inherit; text-decoration: none; }
.fc-topic h2 a:hover { color: var(--accent); }
.visually-hidden { position: absolute; width: 1px; height: 1px; overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; }
button.btn { font: 600 16px/1.2 var(--sans); cursor: pointer; }
.quiz { margin: 24px 0; padding: 22px; background: var(--panel); border: 1px solid var(--line); border-radius: 14px; }
.quiz[hidden] { display: none; }
.quiz__meta { margin: 0; font: 13px/1.4 var(--mono); color: var(--ink-faint); }
.quiz__card { min-height: 160px; margin: 12px 0 16px; font-size: 1.15rem; }
.quiz__q { font-weight: 600; }
.quiz__a { margin-top: 14px; padding-top: 14px; border-top: 1px dashed var(--line); color: var(--ink-soft); }
.quiz__btns { display: flex; flex-wrap: wrap; gap: 10px; }
.quiz__btns button { padding: 10px 16px; font: 600 15px/1 var(--sans); border-radius: 999px; border: 1.5px solid var(--line); background: var(--panel); color: var(--ink); cursor: pointer; }
.quiz__btns .is-good { border-color: var(--good); color: var(--good); }
.quiz__btns .is-again { border-color: var(--bad); color: var(--bad); }
.fc-topic { margin-top: 2.2em; }
.fc-topic h2 { font-size: 1.3rem; margin: 0 0 4px; }

/* --- Footer ------------------------------------------------------------------------ */
.foot { display: flex; flex-wrap: wrap; justify-content: space-between; gap: 8px 24px; padding: 24px clamp(20px, 4vw, 56px); border-top: 1px solid var(--line); color: var(--ink-faint); font-size: 14px; }
.foot p { margin: 0; }

/* --- Responsive ----------------------------------------------------------------------- */
@media (max-width: 1240px) {
  .doc { grid-template-columns: minmax(0, 1fr); }
  .toc { display: none; }
}
@media (max-width: 900px) {
  .layout { grid-template-columns: minmax(0, 1fr); }
  .top__menu { display: grid; }
  .search { margin-left: 0; }
  .brand span { display: none; }
  .sidebar {
    position: fixed;
    z-index: 50;
    top: var(--top);
    left: 0;
    width: min(320px, 86vw);
    background: var(--bg);
    box-shadow: 10px 0 30px rgba(0, 0, 0, 0.2);
    transform: translateX(-105%);
    transition: transform 0.2s ease;
  }
  .sidebar.is-open { transform: none; }
  .procon { grid-template-columns: 1fr; }
  .main { padding-top: 28px; }
}
@media (max-width: 520px) {
  body { font-size: 16px; }
  .pager { grid-template-columns: 1fr; }
  .pager__next { grid-column: 1; }
}
@media (prefers-reduced-motion: reduce) {
  .sidebar { transition: none; }
}