:root {
  --paper: #f2f5f9;
  --panel: #ffffff;
  --ink: #142033;
  --muted: #55627a;
  --rule: #d5dce8;
  --cobalt: #1f4fd8;
  --cobalt-soft: #e4ebfc;
  --teal: #0b6b5d;
  --teal-soft: #dff3ee;
  --amber: #8a5a00;
  --amber-soft: #fbefd2;
  --rose: #a4221b;
  --code-bg: #e9eef6;
  --serif: "Literata", Georgia, "Times New Roman", serif;
  --sans: "IBM Plex Sans", system-ui, -apple-system, "Segoe UI", sans-serif;
  --mono: "IBM Plex Mono", ui-monospace, SFMono-Regular, Menlo, monospace;
  --nav-w: 292px;
  --top-h: 56px;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --paper: #0f1623; --panel: #162033; --ink: #e5eaf3; --muted: #9aa8c0; --rule: #28354d;
    --cobalt: #7fa3ff; --cobalt-soft: #1b2a4d; --teal: #5fd0bb; --teal-soft: #123a35;
    --amber: #f0c36a; --amber-soft: #3a2d10; --rose: #ff8c84; --code-bg: #1b2740;
  }
}
:root[data-theme="dark"] {
  --paper: #0f1623; --panel: #162033; --ink: #e5eaf3; --muted: #9aa8c0; --rule: #28354d;
  --cobalt: #7fa3ff; --cobalt-soft: #1b2a4d; --teal: #5fd0bb; --teal-soft: #123a35;
  --amber: #f0c36a; --amber-soft: #3a2d10; --rose: #ff8c84; --code-bg: #1b2740;
}

* { box-sizing: border-box; }
html { scroll-padding-top: calc(var(--top-h) + 16px); }
body {
  margin: 0; background: var(--paper); color: var(--ink);
  font-family: var(--sans); font-size: 16px; line-height: 1.55;
  -webkit-font-smoothing: antialiased;
}
a { color: var(--cobalt); text-underline-offset: 2px; }
:focus-visible { outline: 2px solid var(--cobalt); outline-offset: 2px; border-radius: 3px; }
.skip { position: absolute; left: -999px; top: 8px; background: var(--cobalt); color: #fff; padding: 8px 12px; border-radius: 6px; z-index: 100; }
.skip:focus { left: 8px; }

/* top bar */
.top {
  position: sticky; top: 0; z-index: 30; height: var(--top-h);
  display: flex; align-items: center; gap: 12px; padding: 0 16px;
  background: color-mix(in srgb, var(--paper) 92%, transparent); backdrop-filter: blur(8px);
  border-bottom: 1px solid var(--rule);
}
.brand { font-weight: 600; color: var(--ink); text-decoration: none; letter-spacing: -0.01em; margin-right: auto; }
.icon-btn, .search-btn {
  display: inline-flex; align-items: center; gap: 8px; cursor: pointer;
  background: var(--panel); color: var(--ink); border: 1px solid var(--rule); border-radius: 8px;
  font: inherit; font-size: 14px; height: 36px;
}
.icon-btn { width: 36px; justify-content: center; padding: 0; }
.search-btn { padding: 0 10px; color: var(--muted); min-width: 240px; }
.search-btn kbd { margin-left: auto; font-family: var(--mono); font-size: 12px; border: 1px solid var(--rule); border-radius: 4px; padding: 0 5px; }
.menu-btn { display: none; }

/* layout */
.shell { display: grid; grid-template-columns: var(--nav-w) minmax(0, 1fr) 240px; max-width: 1560px; margin: 0 auto; }
.side {
  position: sticky; top: var(--top-h); height: calc(100vh - var(--top-h)); overflow-y: auto;
  padding: 20px 12px 40px 20px; border-right: 1px solid var(--rule); font-size: 14.5px;
}
.side details.grp { margin-top: 14px; }
.side details.grp > summary { list-style: none; cursor: pointer; font-size: 13px; font-weight: 600; color: var(--muted); padding: 4px 10px; border-radius: 6px; display: flex; align-items: center; gap: 6px; }
.side details.grp > summary::-webkit-details-marker { display: none; }
.side details.grp > summary::before { content: '▸'; font-size: 11px; transition: transform .15s; }
.side details.grp[open] > summary::before { transform: rotate(90deg); }
.side details.grp > summary:hover { color: var(--ink); }
.side a {
  display: flex; align-items: baseline; gap: 8px; padding: 6px 10px; border-radius: 8px;
  color: var(--ink); text-decoration: none; line-height: 1.35;
}
.side a:hover { background: var(--cobalt-soft); }
.side a[aria-current="page"] { background: var(--cobalt); color: #fff; }
.side a .tick { width: 14px; flex: none; font-size: 12px; color: var(--teal); }
.side a[aria-current="page"] .tick { color: #fff; }
.side .meter { margin: 0 10px 14px; height: 6px; border-radius: 3px; background: var(--rule); overflow: hidden; }
.side .meter i { display: block; height: 100%; background: var(--teal); width: 0; transition: width .3s; }
.side .meter-label { margin: 0 10px 6px; font-size: 13px; color: var(--muted); }

main { padding: 36px clamp(16px, 4vw, 56px) 80px; min-width: 0; outline: none; }
.toc {
  position: sticky; top: var(--top-h); align-self: start; max-height: calc(100vh - var(--top-h));
  overflow-y: auto; padding: 36px 16px 40px 8px; font-size: 13px;
}
.toc p { margin: 0 0 8px; font-weight: 600; color: var(--muted); }
.toc a { display: block; padding: 3px 0 3px 10px; color: var(--muted); text-decoration: none; border-left: 2px solid var(--rule); line-height: 1.35; }
.toc a.l3 { padding-left: 22px; }
.toc a:hover { color: var(--ink); }
.toc a.on { color: var(--cobalt); border-left-color: var(--cobalt); }

/* reading column */
.prose { max-width: 74ch; font-family: var(--serif); font-size: 17px; line-height: 1.7; font-optical-sizing: auto; }
.prose h1 { font-family: var(--sans); font-size: clamp(30px, 4vw, 42px); line-height: 1.1; letter-spacing: -0.025em; margin: 0 0 20px; font-weight: 600; }
.prose h2 { font-family: var(--sans); font-size: 26px; letter-spacing: -0.015em; line-height: 1.2; margin: 2.2em 0 .5em; padding-top: 1em; border-top: 1px solid var(--rule); font-weight: 600; }
.prose hr + h2 { border-top: 0; padding-top: 0; margin-top: 1.2em; }
.prose h3 { font-family: var(--sans); font-size: 19px; line-height: 1.3; margin: 1.8em 0 .4em; font-weight: 600; }
.prose h4 { font-family: var(--sans); font-size: 16px; margin: 1.4em 0 .3em; font-weight: 600; }
.prose h1 + p, .prose h1 + p em { color: var(--muted); }
.prose p, .prose li { max-width: 74ch; }
.prose li { margin: .25em 0; }
.prose ul, .prose ol { padding-left: 1.4em; }
.prose strong { font-weight: 600; }
.prose hr { border: 0; border-top: 1px solid var(--rule); margin: 2.4em 0; }
.prose img { max-width: 100%; height: auto; background: #fff; border-radius: 10px; border: 1px solid var(--rule); }
.prose blockquote {
  margin: 1.3em 0; padding: 12px 16px; border-left: 4px solid var(--amber);
  background: var(--amber-soft); border-radius: 0 8px 8px 0; font-size: 16px;
}
.prose blockquote p { margin: .4em 0; }
.prose code { font-family: var(--mono); font-size: .85em; background: var(--code-bg); padding: .1em .35em; border-radius: 4px; }
.prose pre {
  font-family: var(--mono); font-size: 13.5px; line-height: 1.55; background: var(--code-bg);
  border: 1px solid var(--rule); border-radius: 10px; padding: 14px 16px; overflow-x: auto; max-width: 100%;
}
.prose pre code { background: none; padding: 0; font-size: inherit; }
.table-wrap { overflow-x: auto; margin: 1.2em 0; max-width: 100%; border: 1px solid var(--rule); border-radius: 10px; background: var(--panel); }
.prose table { border-collapse: collapse; font-family: var(--sans); font-size: 14.5px; line-height: 1.45; width: 100%; }
.prose th, .prose td { text-align: left; vertical-align: top; padding: 9px 12px; border-bottom: 1px solid var(--rule); min-width: 90px; }
.prose th { font-weight: 600; background: var(--cobalt-soft); position: sticky; top: 0; }
.prose tr:last-child td { border-bottom: 0; }
.prose td code { white-space: nowrap; }
.prose details { margin: 1.2em 0; border: 1px solid var(--rule); border-radius: 10px; background: var(--panel); padding: 0 16px; }
.prose details[open] { border-color: var(--teal); }
.prose summary { cursor: pointer; font-family: var(--sans); font-weight: 600; padding: 12px 0; color: var(--teal); }
.prose details[open] > summary { border-bottom: 1px solid var(--rule); margin-bottom: 8px; }
.prose li.task { list-style: none; margin-left: -1.4em; }
.prose li.task label { display: flex; gap: 10px; align-items: baseline; cursor: pointer; }
.prose li.task input { transform: translateY(2px); accent-color: var(--teal); width: 16px; height: 16px; flex: none; }
.prose li.task:has(input:checked) { color: var(--muted); text-decoration: line-through; text-decoration-color: var(--rule); }
.book-intro { font-family: var(--sans); }

/* prep dashboard (start page) */
.prep { font-family: var(--sans); background: var(--panel); border: 1px solid var(--rule); border-radius: 14px; padding: 18px 20px; margin: 0 0 32px; display: grid; gap: 14px; grid-template-columns: repeat(auto-fit, minmax(190px, 1fr)); }
.prep .cell { display: grid; gap: 4px; align-content: start; }
.prep .big { font-size: 34px; line-height: 1; font-weight: 600; letter-spacing: -0.03em; color: var(--cobalt); }
.prep .lbl { font-size: 13.5px; color: var(--muted); }
.prep input[type=date] { font: inherit; font-size: 14px; background: var(--paper); color: var(--ink); border: 1px solid var(--rule); border-radius: 6px; padding: 3px 6px; width: fit-content; }
.prep a.cta, .btn {
  display: inline-block; background: var(--cobalt); color: #fff; text-decoration: none; border: 0; cursor: pointer;
  font: 500 14.5px var(--sans); padding: 8px 14px; border-radius: 8px;
}
.btn.ghost { background: var(--panel); color: var(--ink); border: 1px solid var(--rule); }
.btn.good { background: var(--teal); }
.btn:disabled { opacity: .5; cursor: default; }

/* pager */
.pager { max-width: 74ch; display: flex; gap: 12px; flex-wrap: wrap; margin-top: 48px; padding-top: 20px; border-top: 1px solid var(--rule); font-family: var(--sans); }
.pager .grow { flex: 1; }
.pager a { display: block; border: 1px solid var(--rule); background: var(--panel); border-radius: 10px; padding: 10px 14px; text-decoration: none; color: var(--ink); min-width: 200px; }
.pager a small { display: block; color: var(--muted); font-size: 12.5px; }
.pager a:hover { border-color: var(--cobalt); }
.pager .next { text-align: right; margin-left: auto; }

/* drill */
.drill { font-family: var(--sans); max-width: 860px; }
.drill h1 { margin-bottom: 8px; }
.tabs { display: flex; gap: 6px; margin: 18px 0; flex-wrap: wrap; }
.tabs button { font: 500 14px var(--sans); border: 1px solid var(--rule); background: var(--panel); color: var(--ink); border-radius: 999px; padding: 6px 14px; cursor: pointer; }
.tabs button[aria-pressed="true"] { background: var(--ink); color: var(--paper); border-color: var(--ink); }
.decks { display: flex; gap: 6px; flex-wrap: wrap; margin-bottom: 18px; }
.decks button { font: 14px var(--sans); border: 1px solid var(--rule); background: transparent; color: var(--muted); border-radius: 6px; padding: 4px 10px; cursor: pointer; }
.decks button[aria-pressed="true"] { background: var(--cobalt-soft); color: var(--cobalt); border-color: var(--cobalt); }
.card {
  position: relative; min-height: 260px; background: var(--panel); border: 1px solid var(--rule); border-radius: 16px;
  padding: 28px 30px; display: flex; flex-direction: column; justify-content: center; cursor: pointer;
  box-shadow: 0 1px 0 var(--rule), 0 8px 0 -4px var(--panel), 0 9px 0 -4px var(--rule);
}
.card .deck { position: absolute; top: 14px; left: 20px; font-size: 13px; color: var(--muted); }
.card .count { position: absolute; top: 14px; right: 20px; font-size: 13px; color: var(--muted); font-family: var(--mono); }
.card .q { font-family: var(--serif); font-size: clamp(21px, 3vw, 27px); line-height: 1.35; }
.card .a { font-family: var(--serif); font-size: 18.5px; line-height: 1.6; border-top: 2px solid var(--teal); padding-top: 16px; margin-top: 18px; }
.card .hint { position: absolute; bottom: 12px; left: 0; right: 0; text-align: center; font-size: 13px; color: var(--muted); }
.card.known { border-color: var(--teal); }
.controls { display: flex; gap: 8px; margin-top: 16px; flex-wrap: wrap; align-items: center; }
.controls .sp { flex: 1; }
.status { font-size: 14px; color: var(--muted); }
.mock { background: var(--panel); border: 1px solid var(--rule); border-radius: 16px; padding: 26px 28px; }
.mock .kind { font-size: 14px; color: var(--cobalt); font-weight: 600; }
.mock .prompt { font-family: var(--serif); font-size: clamp(20px, 2.8vw, 25px); line-height: 1.4; margin: 10px 0 18px; }
.mock .timer { font-family: var(--mono); font-size: 44px; letter-spacing: -0.03em; }
.mock .timer.late { color: var(--rose); }
.mock ul { padding-left: 1.2em; color: var(--muted); font-size: 15px; margin: 14px 0 0; }

/* search */
.search-overlay { position: fixed; inset: 0; background: rgba(10, 16, 28, .55); z-index: 60; display: flex; justify-content: center; align-items: flex-start; padding: 10vh 16px 0; }
.search-overlay[hidden] { display: none; }
.search-box { width: min(680px, 100%); background: var(--panel); border: 1px solid var(--rule); border-radius: 14px; overflow: hidden; box-shadow: 0 20px 60px rgba(0,0,0,.35); }
.search-box input { width: 100%; font: 17px var(--sans); padding: 16px 18px; border: 0; border-bottom: 1px solid var(--rule); background: transparent; color: var(--ink); outline: none; }
.search-results { max-height: 56vh; overflow-y: auto; }
.search-results a { display: block; padding: 11px 18px; text-decoration: none; color: var(--ink); border-bottom: 1px solid var(--rule); font-size: 14.5px; }
.search-results a:hover, .search-results a.sel { background: var(--cobalt-soft); }
.search-results b { display: block; font-weight: 600; }
.search-results small { color: var(--muted); display: block; margin-top: 2px; line-height: 1.4; }
.search-results mark { background: var(--amber-soft); color: inherit; border-radius: 2px; }
.search-results .none { padding: 18px; color: var(--muted); }

.scrim { display: none; }

@media (max-width: 1240px) {
  .shell { grid-template-columns: var(--nav-w) minmax(0, 1fr); }
  .toc { display: none; }
}
@media (max-width: 860px) {
  .shell { grid-template-columns: minmax(0, 1fr); }
  .menu-btn { display: inline-flex; }
  .search-btn { min-width: 0; } .search-btn span, .search-btn kbd { display: none; } .search-btn { width: 36px; justify-content: center; padding: 0; }
  .side { position: fixed; z-index: 50; left: 0; top: 0; bottom: 0; height: auto; width: min(86vw, 320px); background: var(--paper); transform: translateX(-102%); transition: transform .2s; padding-top: 72px; }
  body.nav-open .side { transform: none; box-shadow: 8px 0 40px rgba(0,0,0,.3); }
  body.nav-open .scrim { display: block; position: fixed; inset: 0; background: rgba(10,16,28,.45); z-index: 40; }
  main { padding-top: 24px; }
  .prose { font-size: 16.5px; }
}
@media (prefers-reduced-motion: reduce) { * { transition: none !important; scroll-behavior: auto !important; } }
@media print {
  .top, .side, .toc, .pager { display: none !important; }
  .shell { display: block; } main { padding: 0; }
  .prose details { display: block; } .prose details > * { display: block; }
}

.side details.grp > summary em { margin-left: auto; font-style: normal; font-weight: 500; font-size: 12px; color: var(--muted); font-family: var(--mono); }
.side a .num { font-family: var(--mono); font-weight: 500; font-size: 12.5px; opacity: .75; margin-right: 2px; }
.side a .fast { font-style: normal; font-size: 11px; border: 1px solid currentColor; border-radius: 4px; padding: 0 4px; margin-left: 4px; opacity: .65; }
.prose h1 + p em { font-family: var(--sans); font-size: 14px; }