:root {
  --bg: #f4f2ec;
  --surface: #ffffff;
  --surface-2: #ece9e1;
  --text: #1c1c22;
  --muted: #5e5d66;
  --line: #d6d2c8;
  --accent: #c2410c;
  --accent-text: #ffffff;
  --focus: #2563eb;
  --ok: #15803d;
  --bad: #dc2626;
  --sel: #f5b400;
  --c-R: #d62828;
  --c-G: #1f9a48;
  --c-B: #1d4ed8;
  --c-W: #f4f4ef;
  --scene-bg: radial-gradient(ellipse at 50% 35%, #ffffff 0%, #e6e2d8 75%);
  --radius: 14px;
  color-scheme: light;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #111217;
    --surface: #1b1c23;
    --surface-2: #262833;
    --text: #ecebf2;
    --muted: #a3a2ad;
    --line: #34363f;
    --accent: #f97316;
    --accent-text: #1a0d02;
    --focus: #60a5fa;
    --ok: #4ade80;
    --bad: #f87171;
    --scene-bg: radial-gradient(ellipse at 50% 35%, #2a2c38 0%, #121318 75%);
    color-scheme: dark;
  }
}
:root[data-theme="dark"] {
  --bg: #111217;
  --surface: #1b1c23;
  --surface-2: #262833;
  --text: #ecebf2;
  --muted: #a3a2ad;
  --line: #34363f;
  --accent: #f97316;
  --accent-text: #1a0d02;
  --focus: #60a5fa;
  --ok: #4ade80;
  --bad: #f87171;
  --scene-bg: radial-gradient(ellipse at 50% 35%, #2a2c38 0%, #121318 75%);
  color-scheme: dark;
}

*, *::before, *::after { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font: 16px/1.5 system-ui, -apple-system, "Segoe UI", Roboto, sans-serif;
  overflow-x: hidden;
  padding: env(safe-area-inset-top) env(safe-area-inset-right) env(safe-area-inset-bottom) env(safe-area-inset-left);
}
h1, h2, h3 { line-height: 1.2; margin: 0; }
a { color: var(--accent); }
:focus-visible { outline: 3px solid var(--focus); outline-offset: 2px; }
.sr-only {
  position: absolute; width: 1px; height: 1px; padding: 0; margin: -1px;
  overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; border: 0;
}
.skip { position: absolute; left: -999px; top: 8px; z-index: 10; background: var(--surface); padding: 8px 12px; }
.skip:focus { left: 16px; }

/* ---------- buttons ---------- */
button {
  font: inherit;
  min-height: 44px;
  min-width: 44px;
  border-radius: 12px;
  border: 1px solid var(--line);
  background: var(--surface-2);
  color: var(--text);
  padding: 8px 12px;
  cursor: pointer;
  touch-action: manipulation;
  -webkit-tap-highlight-color: transparent;
}
button:active:not(:disabled) { transform: scale(0.97); }
button:disabled { opacity: 0.4; cursor: not-allowed; }
button.primary { background: var(--accent); color: var(--accent-text); border-color: transparent; font-weight: 650; }
button.ghost { background: transparent; }
button.link { background: none; border: none; color: var(--accent); text-decoration: underline; padding: 0 4px; }

/* ---------- top bar ---------- */
.topbar {
  display: flex; flex-wrap: wrap; align-items: center; justify-content: space-between; gap: 8px;
  padding: 10px 16px;
  border-bottom: 1px solid var(--line);
}
.topbar h1 { font-size: 1.3rem; letter-spacing: -0.01em; }
.topbar nav { display: flex; align-items: center; gap: 4px; flex-wrap: wrap; }
.topbar nav a {
  display: inline-flex; align-items: center; min-height: 44px; padding: 0 10px;
  color: var(--text); text-decoration: none; border-radius: 10px;
}
.topbar nav a[aria-current="page"] { font-weight: 700; text-decoration: underline; text-underline-offset: 4px; }

/* ---------- layout ---------- */
.game {
  display: grid; gap: 12px; padding: 12px 16px 16px;
  max-width: 1200px; margin: 0 auto;
  grid-template-columns: minmax(0, 1fr);
}
.card { background: var(--surface); border: 1px solid var(--line); border-radius: var(--radius); padding: 14px; }
.card h2 { font-size: 1.05rem; }
.sub { color: var(--muted); font-size: 0.88rem; margin: 4px 0 10px; }

.stats {
  display: grid; grid-template-columns: repeat(5, minmax(0, 1fr)); gap: 6px;
  text-align: center;
}
.stats > div { background: var(--surface); border: 1px solid var(--line); border-radius: 10px; padding: 6px 2px; min-width: 0; }
.stats .k { display: block; font-size: 0.7rem; text-transform: uppercase; letter-spacing: 0.06em; color: var(--muted); }
.stats .v { display: block; font-size: 1.2rem; font-weight: 700; font-variant-numeric: tabular-nums; }
.stats .best { grid-column: 1 / -1; display: flex; justify-content: center; align-items: baseline; gap: 8px; }
.stats .best .v { font-size: 0.95rem; }

.viewer { position: relative; }
#scene {
  position: relative;
  height: min(58vh, 520px); min-height: 320px;
  border-radius: var(--radius);
  background: var(--scene-bg);
  border: 1px solid var(--line);
  overflow: hidden;
}
#scene canvas { display: block; width: 100%; height: 100%; touch-action: none; cursor: grab; }
#scene .fallback { display: none; padding: 24px; color: var(--muted); }
#scene.no3d { height: auto; min-height: 0; }
#scene.no3d .fallback { display: block; }
.tip { text-align: center; color: var(--muted); font-size: 0.8rem; margin: 6px 0 0; }
.solved {
  position: absolute; left: 12px; right: 12px; bottom: 44px; margin: 0;
  background: var(--ok); color: #fff; font-weight: 700; text-align: center;
  padding: 8px 12px; border-radius: 10px;
}

/* ---------- four sides ---------- */
.sides-table { width: 100%; border-collapse: separate; border-spacing: 6px; table-layout: fixed; }
.sides-table th { font-size: 0.85rem; font-weight: 650; }
.sides-table thead th:first-child, .sides-table tbody th { width: 2.2rem; color: var(--muted); }
.sides-table thead th.ok { color: var(--ok); }
.sides-table thead th.bad { color: var(--bad); }
.sides-table .mark { margin-left: 4px; }
.sides-table tr.is-selected th { color: var(--text); }
.sides-table tr.is-selected th::before { content: "▸"; color: var(--sel); }
.sides-table td { padding: 0; text-align: center; }
.sq {
  display: flex; align-items: center; justify-content: center;
  aspect-ratio: 1; max-height: 64px; margin: 0 auto; width: 100%; max-width: 64px;
  border-radius: 9px; border: 2px solid rgba(0, 0, 0, 0.25);
  font-weight: 800; font-size: 1.1rem; color: transparent;
}
.letters .sq { color: #fff; }
.letters .sq.c-W { color: #1a1a1a; }
.c-R { background: var(--c-R); }
.c-G { background: var(--c-G); }
.c-B { background: var(--c-B); }
.c-W { background: var(--c-W); }
.sq.dup { border-color: var(--bad); box-shadow: 0 0 0 3px var(--bad); position: relative; }
.sq.dup::after {
  content: "!"; position: absolute; top: -8px; right: -8px;
  width: 18px; height: 18px; border-radius: 50%;
  background: var(--bad); color: #fff; font-size: 12px; line-height: 18px;
}

/* ---------- controls ---------- */
.chips { display: grid; grid-template-columns: repeat(4, minmax(0, 1fr)); gap: 6px; }
.chip[aria-pressed="true"] { background: var(--sel); color: #1a1400; border-color: var(--sel); font-weight: 700; }
.selected { text-align: center; color: var(--muted); font-size: 0.88rem; margin: 8px 0; }
.moves { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 8px; }
.moves button {
  min-height: 60px; display: flex; align-items: center; justify-content: center; gap: 8px;
  font-weight: 600; font-size: 0.95rem; line-height: 1.15;
}
.moves .ico { font-size: 1.5rem; line-height: 1; }
.moves button.pulse { animation: pulse 0.9s ease-out 3; }
@keyframes pulse {
  0% { box-shadow: 0 0 0 0 var(--sel); }
  100% { box-shadow: 0 0 0 12px transparent; }
}
.swaps { display: grid; grid-template-columns: 1fr 1fr; gap: 8px; margin-top: 8px; }
.actions { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 8px; margin-top: 12px; }
.solve { width: 100%; margin-top: 8px; }
.solution { margin-top: 12px; padding: 12px; border-radius: 12px; background: var(--surface-2); }
.solution h3 { font-size: 1rem; }
.steps { margin: 4px 0 10px; padding-left: 28px; }
.steps li { padding: 4px 6px; border-radius: 6px; }
.steps li.current { background: var(--sel); color: #1a1400; font-weight: 700; }
.steps li.done { color: var(--muted); text-decoration: line-through; }
.solution-actions { display: grid; grid-template-columns: 1fr 1fr; gap: 8px; }
.hint { min-height: 1.5em; margin: 8px 0 0; font-weight: 600; text-align: center; }
.hint:empty { min-height: 0; margin: 0; }
.toggle { display: flex; align-items: center; gap: 10px; margin-top: 10px; min-height: 44px; cursor: pointer; }
.toggle input { width: 24px; height: 24px; accent-color: var(--accent); }

.foot { max-width: 1200px; margin: 0 auto; padding: 0 16px 24px; color: var(--muted); font-size: 0.85rem; text-align: center; }

/* ---------- dialogs ---------- */
dialog {
  border: 1px solid var(--line); border-radius: 18px; padding: 20px;
  background: var(--surface); color: var(--text);
  width: min(92vw, 560px); max-height: 92vh; overflow: auto; text-align: center;
}
dialog::backdrop { background: rgba(0, 0, 0, 0.55); }
dialog h2 { font-size: 1.4rem; margin-bottom: 8px; }
dialog .row { display: flex; gap: 8px; justify-content: center; flex-wrap: wrap; margin-top: 14px; }
dialog .big { font-size: 1.3rem; margin: 6px 0; }
.intro { padding: 32px 24px 24px; cursor: pointer; }
.intro::backdrop { background: rgba(0, 0, 0, 0.8); }
.intro-cubes { display: flex; justify-content: center; gap: 8px; margin-bottom: 18px; }
.intro-cubes span {
  width: 34px; height: 34px; border-radius: 8px; border: 3px solid #16171c;
  animation: intro-bob 1.6s ease-in-out infinite;
}
.intro-cubes span:nth-child(2) { animation-delay: 0.15s; }
.intro-cubes span:nth-child(3) { animation-delay: 0.3s; }
.intro-cubes span:nth-child(4) { animation-delay: 0.45s; }
@keyframes intro-bob { 50% { transform: translateY(-8px) rotate(8deg); } }
.intro-kicker { margin: 0; color: var(--muted); text-transform: uppercase; letter-spacing: 0.12em; font-size: 0.8rem; font-weight: 650; }
dialog.intro h2 { font-size: clamp(1.8rem, 7vw, 2.4rem); margin: 6px 0 8px; }
.intro-sub { font-size: 1.1rem; margin: 0 0 22px; }
.intro button { min-width: 200px; min-height: 52px; font-size: 1.1rem; }
.intro-tip { margin: 12px 0 0; color: var(--muted); font-size: 0.85rem; }
.record { color: var(--ok); font-weight: 700; min-height: 1.5em; }
#win-dialog { position: fixed; overflow: hidden; }
#confetti { position: absolute; inset: 0; pointer-events: none; overflow: hidden; }
#confetti span {
  position: absolute; top: -12px; width: 9px; height: 14px; border-radius: 2px;
  animation-name: fall; animation-timing-function: linear; animation-fill-mode: forwards;
}
@keyframes fall { to { transform: translateY(110vh) rotate(720deg); } }
.qr { display: flex; justify-content: center; padding: 14px; background: #fff; border-radius: 12px; }
.qr img, .qr canvas { max-width: 100%; height: auto; }
.url { word-break: break-all; font-size: 0.85rem; color: var(--muted); }
.tests { text-align: left; padding-left: 18px; }
.tests .pass { color: var(--ok); }
.tests .fail { color: var(--bad); font-weight: 700; }

@media (prefers-reduced-motion: reduce) {
  #confetti { display: none; }
  .moves button.pulse { animation: none; outline: 3px solid var(--sel); }
}

/* ---------- wider screens ---------- */
@media (min-width: 900px) {
  .game {
    grid-template-columns: minmax(0, 1.25fr) minmax(0, 1fr);
    grid-template-areas: "stats stats" "viewer sides" "viewer controls";
    align-items: start;
  }
  .stats { grid-area: stats; grid-template-columns: repeat(5, minmax(0, 1fr)) minmax(0, 2fr); }
  .stats .best { grid-column: auto; flex-direction: column; align-items: center; gap: 0; }
  .viewer { grid-area: viewer; position: sticky; top: 12px; }
  #scene { height: min(78vh, 720px); }
  .sides { grid-area: sides; }
  .controls { grid-area: controls; }
  .moves { grid-template-columns: repeat(3, minmax(0, 1fr)); }
  .actions { grid-template-columns: repeat(4, minmax(0, 1fr)); }
}

/* ================= math page ================= */
.article { max-width: 760px; margin: 0 auto; padding: 16px 16px 40px; }
.article section { margin: 0 0 18px; }
.article h2 { font-size: 1.35rem; margin-bottom: 8px; }
.article h3 { font-size: 1.05rem; margin: 14px 0 6px; }
.article p, .article li { max-width: 68ch; }
.lead { font-size: 1.08rem; color: var(--muted); }
.eq {
  display: block; overflow-x: auto; background: var(--surface-2); border-radius: 10px;
  padding: 10px 12px; margin: 8px 0; font-variant-numeric: tabular-nums; font-weight: 600;
}
.fact { border-left: 4px solid var(--accent); padding: 4px 0 4px 12px; margin: 10px 0; }
.num-table { width: 100%; border-collapse: collapse; font-variant-numeric: tabular-nums; }
.num-table th, .num-table td { border-bottom: 1px solid var(--line); padding: 8px 6px; text-align: left; vertical-align: top; }
.num-table td.n { text-align: right; white-space: nowrap; font-weight: 650; }
.table-wrap { overflow-x: auto; }
@media (max-width: 560px) {
  .num-table tr { display: block; border-bottom: 1px solid var(--line); padding: 6px 0; }
  .num-table td { display: block; border: none; padding: 2px 0; }
  .num-table td.n { text-align: left; white-space: normal; }
}
.graph-wrap { background: var(--surface-2); border-radius: 12px; padding: 8px; }
.graph-wrap svg { display: block; width: 100%; height: auto; max-height: 460px; }
.graph-buttons { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 8px; margin: 10px 0; }
.graph-buttons button[aria-pressed="true"] { background: var(--accent); color: var(--accent-text); border-color: transparent; }
.legend { display: flex; flex-wrap: wrap; gap: 6px 14px; font-size: 0.88rem; margin: 6px 0; padding: 0; list-style: none; }
.legend i { display: inline-block; width: 22px; height: 5px; border-radius: 3px; vertical-align: middle; margin-right: 6px; }
.svg-vertex-text { font: 700 22px system-ui, sans-serif; }
.svg-edge { fill: none; stroke-width: 4; stroke-linecap: round; transition: opacity 0.25s, stroke-width 0.25s; }
.svg-label { font: 700 13px system-ui, sans-serif; fill: #fff; }
.graph-wrap.dim .edge-group { opacity: 0.12; }
.graph-wrap.dim .edge-group.on { opacity: 1; }
.graph-wrap.dim .edge-group.on .svg-edge { stroke-width: 7; }
.edge-group.on.fb .svg-edge { stroke-dasharray: none; }
.edge-group.on.lr .svg-edge { stroke-dasharray: 10 6; }
.quiz details {
  background: var(--surface); border: 1px solid var(--line); border-radius: 12px;
  padding: 0 14px; margin: 8px 0;
}
.quiz summary { cursor: pointer; font-weight: 650; min-height: 44px; display: flex; align-items: center; padding: 10px 0; }
.quiz details p { margin-top: 0; }
.cube-strip { display: grid; grid-template-columns: repeat(4, minmax(0, 1fr)); gap: 8px; }
.net { display: grid; grid-template-columns: repeat(4, 1fr); gap: 2px; max-width: 120px; margin: 0 auto 4px; }
.net span {
  aspect-ratio: 1; border-radius: 3px; display: flex; align-items: center; justify-content: center;
  font-size: 0.7rem; font-weight: 800; color: #fff; border: 1px solid rgba(0,0,0,0.25);
}
.net span.c-W { color: #1a1a1a; }
.net span.blank { background: transparent; border: none; }
.cube-strip figcaption { text-align: center; font-size: 0.85rem; color: var(--muted); }