:root {
  --bg: #f7f6f2; --panel: #ffffff; --ink: #1c1d21; --muted: #6a6d75; --line: #e3e1da;
  --accent: #2f5bd3; --accent-soft: #e8eefc; --trap: #b4431f; --trap-soft: #fbeee8;
  --idea: #1f7a4d; --idea-soft: #e7f4ec; --ans: #6a3fb5; --ans-soft: #f1ebfb;
  --radius: 10px; --shadow: 0 1px 2px rgba(0,0,0,.05), 0 2px 8px rgba(0,0,0,.04);
  --serif: "Iowan Old Style", "Charter", "PT Serif", Georgia, serif;
  --sans: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  color-scheme: light;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #15161a; --panel: #1d1f24; --ink: #e8e8ea; --muted: #9a9ca4; --line: #2e3037;
    --accent: #7c9cff; --accent-soft: #232b45; --trap: #ff9a76; --trap-soft: #3a241d;
    --idea: #6fd3a0; --idea-soft: #1b3127; --ans: #c4a5ff; --ans-soft: #2c2440; color-scheme: dark;
  }
}
:root[data-theme="dark"] {
  --bg: #15161a; --panel: #1d1f24; --ink: #e8e8ea; --muted: #9a9ca4; --line: #2e3037;
  --accent: #7c9cff; --accent-soft: #232b45; --trap: #ff9a76; --trap-soft: #3a241d;
  --idea: #6fd3a0; --idea-soft: #1b3127; --ans: #c4a5ff; --ans-soft: #2c2440; color-scheme: dark;
}
* { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body { margin: 0; background: var(--bg); color: var(--ink); font: 16px/1.55 var(--sans); }
a { color: var(--accent); text-decoration: none; }
a:hover { text-decoration: underline; }
h1, h2, h3 { line-height: 1.25; margin: 0 0 .5em; }
h1 { font-size: 1.7rem; } h2 { font-size: 1.25rem; } h3 { font-size: 1.05rem; }

.top { position: sticky; top: 0; z-index: 10; display: flex; gap: 18px; align-items: center;
  padding: 10px 24px; background: color-mix(in srgb, var(--panel) 92%, transparent);
  backdrop-filter: blur(8px); border-bottom: 1px solid var(--line); flex-wrap: wrap; }
.brand { font: 600 1.15rem var(--serif); color: var(--ink); white-space: nowrap; }
.brand:hover { text-decoration: none; }
.top nav { display: flex; gap: 4px; flex-wrap: wrap; }
.top nav a { padding: 6px 10px; border-radius: 8px; color: var(--muted); font-size: .95rem; }
.top nav a:hover { background: var(--accent-soft); color: var(--ink); text-decoration: none; }
.top nav a.on { color: var(--accent); background: var(--accent-soft); }
.qsearch { margin-left: auto; flex: 0 1 280px; }
.qsearch input, .filters input, .filters select { width: 100%; padding: 7px 10px; border: 1px solid var(--line);
  border-radius: 8px; background: var(--panel); color: var(--ink); font: inherit; font-size: .93rem; }
.theme { border: 1px solid var(--line); background: var(--panel); color: var(--ink); border-radius: 8px;
  width: 34px; height: 34px; cursor: pointer; font-size: 1rem; }

main { max-width: 1180px; margin: 0 auto; padding: 24px; min-height: 70vh; }
.loading { color: var(--muted); }
.foot { text-align: center; color: var(--muted); font-size: .85rem; padding: 24px; border-top: 1px solid var(--line); }

.card { background: var(--panel); border: 1px solid var(--line); border-radius: var(--radius); box-shadow: var(--shadow); }
.hero { padding: 28px; margin-bottom: 22px; }
.hero h1 { font-family: var(--serif); font-size: 2rem; }
.hero p { color: var(--muted); margin: 0 0 16px; max-width: 760px; }
.stats { display: flex; gap: 28px; flex-wrap: wrap; }
.stat b { display: block; font-size: 1.6rem; font-variant-numeric: tabular-nums; }
.stat span { color: var(--muted); font-size: .88rem; }
.btn { display: inline-block; padding: 8px 14px; border-radius: 8px; border: 1px solid var(--line);
  background: var(--panel); color: var(--ink); cursor: pointer; font: inherit; font-size: .93rem; }
.btn:hover { border-color: var(--accent); text-decoration: none; }
.btn.primary { background: var(--accent); color: #fff; border-color: var(--accent); }
.row { display: flex; gap: 10px; flex-wrap: wrap; align-items: center; }

.grid { display: grid; gap: 14px; grid-template-columns: repeat(auto-fill, minmax(260px, 1fr)); }
.tile { padding: 16px 18px; display: block; color: var(--ink); }
.tile:hover { border-color: var(--accent); text-decoration: none; }
.tile h3 { margin-bottom: 4px; }
.tile .sub { color: var(--muted); font-size: .88rem; }
.tile ul { margin: 8px 0 0; padding: 0; list-style: none; font-size: .9rem; }
.tile li { display: flex; justify-content: space-between; gap: 8px; padding: 2px 0; }
.tile li span:last-child { color: var(--muted); font-variant-numeric: tabular-nums; }
section { margin-bottom: 30px; }
.section-h { display: flex; justify-content: space-between; align-items: baseline; margin-bottom: 12px; }

.layout { display: grid; grid-template-columns: 260px 1fr; gap: 22px; align-items: start; }
.filters { padding: 16px; position: sticky; top: 72px; max-height: calc(100vh - 90px); overflow: auto; }
.filters label { display: block; font-size: .8rem; color: var(--muted); margin: 12px 0 4px; text-transform: uppercase; letter-spacing: .04em; }
.filters label:first-child { margin-top: 0; }
.chips { display: flex; flex-wrap: wrap; gap: 6px; }
.chip { padding: 4px 9px; border-radius: 999px; border: 1px solid var(--line); background: var(--panel);
  color: var(--ink); font-size: .84rem; cursor: pointer; user-select: none; }
.chip.on { background: var(--accent); border-color: var(--accent); color: #fff; }
.yr { display: flex; gap: 6px; }
.check { display: flex; gap: 8px; align-items: center; font-size: .9rem; margin-top: 10px; cursor: pointer; }

.results-h { display: flex; justify-content: space-between; align-items: center; margin-bottom: 12px; gap: 10px; flex-wrap: wrap; }
.results-h .muted { color: var(--muted); }
.plist { display: flex; flex-direction: column; gap: 10px; }
.pitem { padding: 14px 16px; display: block; color: var(--ink); }
.pitem:hover { border-color: var(--accent); text-decoration: none; }
.pitem .meta { font-size: .82rem; color: var(--muted); display: flex; gap: 10px; flex-wrap: wrap; align-items: center; }
.pitem h3 { margin: 4px 0 6px; font-family: var(--serif); font-size: 1.08rem; }
.pitem .snip { color: var(--muted); font-size: .9rem; display: -webkit-box; -webkit-line-clamp: 2; -webkit-box-orient: vertical; overflow: hidden; }
.tag { display: inline-block; padding: 1px 8px; border-radius: 999px; font-size: .78rem; background: var(--accent-soft); color: var(--accent); }
.tag.trap { background: var(--trap-soft); color: var(--trap); }
.tag.done { background: var(--idea-soft); color: var(--idea); }
.diff { letter-spacing: 1px; color: var(--accent); font-size: .8rem; }
.more { margin: 16px auto 0; display: block; }

.prob { padding: 26px 30px; }
.prob .crumbs { font-size: .88rem; color: var(--muted); margin-bottom: 8px; }
.prob h1 { font-family: var(--serif); }
.statement { font-family: var(--serif); font-size: 1.08rem; line-height: 1.65; white-space: pre-wrap; overflow-wrap: anywhere; }
.statement .katex-display { white-space: normal; }
.figs { display: flex; flex-wrap: wrap; gap: 12px; margin: 16px 0; }
.figs img { max-width: 100%; border: 1px solid var(--line); border-radius: 8px; background: #fff; }
.box { border-radius: var(--radius); padding: 14px 18px; margin: 16px 0; }
.box h3 { font-size: .85rem; text-transform: uppercase; letter-spacing: .05em; margin-bottom: 8px; }
.box ul { margin: 0; padding-left: 20px; }
.box li + li { margin-top: 4px; }
.box.ideas { background: var(--idea-soft); } .box.ideas h3 { color: var(--idea); }
.box.traps { background: var(--trap-soft); } .box.traps h3 { color: var(--trap); }
.box.answer { background: var(--ans-soft); } .box.answer h3 { color: var(--ans); }
.box .body { white-space: pre-wrap; overflow-wrap: anywhere; }
details.spoil summary { cursor: pointer; font-weight: 600; color: var(--ans); list-style: none; }
details.spoil summary::-webkit-details-marker { display: none; }
details.spoil summary::before { content: "▸ "; }
details.spoil[open] summary::before { content: "▾ "; }
details.spoil .body { margin-top: 10px; }
.links { display: flex; gap: 10px; flex-wrap: wrap; margin: 18px 0 6px; }
.nav2 { display: flex; justify-content: space-between; gap: 10px; margin-top: 18px; flex-wrap: wrap; }
.marks { display: flex; gap: 8px; }
.marks .btn.on { background: var(--idea-soft); border-color: var(--idea); color: var(--idea); }

.traplist { list-style: none; padding: 0; margin: 0; }
.traplist li { padding: 10px 0; border-bottom: 1px solid var(--line); }
.traplist li:last-child { border-bottom: 0; }
.traplist .src { font-size: .82rem; color: var(--muted); margin-left: 6px; }
.mx { border-collapse: collapse; width: 100%; font-size: .9rem; }
.mx th, .mx td { border-bottom: 1px solid var(--line); padding: 6px 8px; text-align: left; vertical-align: top; }
.mx td a { margin-right: 6px; }
.tbl-wrap { overflow-x: auto; }
.empty { padding: 30px; text-align: center; color: var(--muted); }

@media (max-width: 860px) {
  .top { padding: 10px 16px; gap: 10px; }
  .qsearch { flex: 1 1 100%; order: 5; margin: 0; }
  main { padding: 16px; }
  .layout { grid-template-columns: 1fr; }
  .filters { position: static; max-height: none; }
  .prob { padding: 18px 16px; }
  .hero { padding: 20px 16px; }
}
.layout.two { grid-template-columns: 1fr 1fr; }
@media (max-width: 860px) { .layout.two { grid-template-columns: 1fr; } }
.filters label.check { display: flex; text-transform: none; letter-spacing: 0; color: var(--ink); font-size: .9rem; margin-top: 14px; }
.check input { width: auto; }
.fsum { display: none; }
@media (min-width: 861px) { #fbox > summary { display: none; } }
@media (max-width: 860px) {
  .fsum { display: block; cursor: pointer; font-weight: 600; list-style: none; }
  .fsum::-webkit-details-marker { display: none; }
  .fsum::after { content: " ▾"; color: var(--muted); }
  #fbox[open] .fsum { margin-bottom: 12px; }
  .filters { padding: 12px 16px; }
  .top { gap: 8px 10px; }
  .brand span { display: none; }
  .top nav { flex-wrap: nowrap; overflow-x: auto; flex: 1 1 0; min-width: 0; scrollbar-width: none; }
  .top nav a { white-space: nowrap; padding: 6px 8px; }
}