:root {
  --bg: #0f1115;
  --panel: #171a21;
  --panel2: #1e222b;
  --codebg: #0b0d11;
  --line: #2a2f3a;
  --text: #e6e8ee;
  --dim: #98a1b1;
  --accent: #7aa2ff;
  --accent-bg: rgba(122, 162, 255, .14);
  --ok: #4cc38a;
  --ok-bg: rgba(76, 195, 138, .14);
  --bad: #ff6b6b;
  --bad-bg: rgba(255, 107, 107, .13);
  --warn: #f5b84b;
  --warn-bg: rgba(245, 184, 75, .12);
  --mono: ui-monospace, "SF Mono", SFMono-Regular, Menlo, Consolas, monospace;
  --top: env(safe-area-inset-top, 0px);
  --bottom: env(safe-area-inset-bottom, 0px);
  color-scheme: dark;
}

* { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font: 16px/1.5 -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
  -webkit-tap-highlight-color: transparent;
}
a { color: var(--accent); }
h2 { font-size: 1.05rem; margin: 28px 0 10px; color: var(--dim); text-transform: uppercase; letter-spacing: .06em; }
h3 { font-size: 1.05rem; margin: 26px 0 10px; }
p { margin: 8px 0; }
ul { padding-left: 1.25em; }
li { margin: 4px 0; }
code { font-family: var(--mono); font-size: .9em; background: var(--panel2); padding: 1px 5px; border-radius: 5px; white-space: pre-wrap; }
.dim { color: var(--dim); }
.mono { font-family: var(--mono); }
[hidden] { display: none !important; }

/* ---------- chrome ---------- */
#topbar {
  position: sticky; top: 0; z-index: 10;
  display: flex; align-items: center; gap: 6px;
  padding: calc(var(--top) + 8px) 12px 8px;
  background: rgba(15, 17, 21, .94);
  backdrop-filter: blur(10px); -webkit-backdrop-filter: blur(10px);
  border-bottom: 1px solid var(--line);
}
#topbar h1 { font-size: 1.05rem; margin: 0; white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }
#back {
  min-width: 44px; height: 44px; margin-left: -8px;
  background: none; border: 0; color: var(--accent); font-size: 2rem; line-height: 1; cursor: pointer;
}
main {
  max-width: 980px; margin: 0 auto;
  padding: 14px 16px calc(96px + var(--bottom));
}
#tabbar {
  position: fixed; left: 0; right: 0; bottom: 0; z-index: 10;
  display: flex; justify-content: space-around;
  padding: 6px 4px calc(6px + var(--bottom));
  background: rgba(15, 17, 21, .96);
  backdrop-filter: blur(10px); -webkit-backdrop-filter: blur(10px);
  border-top: 1px solid var(--line);
}
#tabbar a {
  flex: 1; display: flex; flex-direction: column; align-items: center; gap: 2px;
  min-height: 48px; padding-top: 4px;
  color: var(--dim); text-decoration: none; font-size: .72rem;
}
#tabbar a.on { color: var(--accent); }
#tabbar svg { width: 24px; height: 24px; fill: none; stroke: currentColor; stroke-width: 1.8; stroke-linejoin: round; stroke-linecap: round; }

/* ---------- building blocks ---------- */
.card {
  background: var(--panel); border: 1px solid var(--line); border-radius: 14px;
  padding: 14px; margin: 10px 0;
}
.btn {
  display: inline-flex; align-items: center; justify-content: center;
  min-height: 48px; padding: 10px 18px; margin: 6px 6px 6px 0;
  background: var(--panel2); color: var(--text);
  border: 1px solid var(--line); border-radius: 12px;
  font: inherit; font-weight: 600; text-decoration: none; cursor: pointer;
}
.btn.primary { background: var(--accent); border-color: var(--accent); color: #0b1020; }
.btn:disabled { opacity: .45; cursor: default; }
.btn.block { display: flex; width: 100%; }
.row { display: flex; flex-wrap: wrap; gap: 0; align-items: center; }
.label { font-size: .75rem; text-transform: uppercase; letter-spacing: .06em; color: var(--dim); margin: 14px 0 6px; }
.badge {
  display: inline-block; padding: 2px 9px; border-radius: 99px;
  font-size: .75rem; font-weight: 600; background: var(--panel2); color: var(--dim); white-space: nowrap;
}
.st-learning { background: var(--warn-bg); color: var(--warn); }
.st-helped { background: var(--accent-bg); color: var(--accent); }
.st-cold { background: var(--ok-bg); color: var(--ok); }
.st-you { background: var(--accent-bg); color: var(--accent); }
.toggle {
  min-height: 44px; padding: 8px 14px; margin: 4px 0 8px;
  background: none; border: 1px dashed var(--line); border-radius: 10px;
  color: var(--accent); font: inherit; font-size: .9rem; cursor: pointer;
}
.verify {
  background: var(--warn-bg); color: var(--warn); border-radius: 10px;
  padding: 8px 12px; font-size: .88rem; margin: 6px 0 12px;
}
.prose p:first-child { margin-top: 0; }

/* ---------- code ---------- */
pre {
  margin: 0; font: 14px/1.55 var(--mono); tab-size: 3;
  white-space: pre; overflow-x: auto; -webkit-overflow-scrolling: touch;
}
pre.code, pre.out {
  background: var(--codebg); border: 1px solid var(--line); border-radius: 10px; padding: 11px 12px;
}
pre.out { color: #e9e3c9; }
.ws { color: #5a6273; }
.k { color: #c792ea; }
.t { color: #ffcb6b; }
.s { color: #c3e88d; }
.n { color: #f78c6c; }
.c { color: #6b7590; font-style: italic; }
.l { color: #ff9cac; }
.f { color: #82aaff; }
.a { color: #89ddff; }
.lines { display: inline-block; min-width: 100%; }
.line { display: block; min-height: 1.55em; }
.line.mine { background: var(--accent-bg); box-shadow: inset 3px 0 0 var(--accent); margin: 0 -12px; padding: 0 12px; }

/* ---------- home ---------- */
.hero-grid { display: grid; gap: 10px; grid-template-columns: 1fr; }
@media (min-width: 640px) { .hero-grid { grid-template-columns: 1fr 1fr; } }
.hero-grid .card { margin: 0; }
.card.go { display: block; text-decoration: none; color: var(--text); }
.card.go b { display: block; font-size: 1.1rem; }
.card.go span { color: var(--dim); font-size: .9rem; }
.duelist a { display: flex; align-items: center; min-height: 44px; text-decoration: none; border-top: 1px solid var(--line); }
.chips { display: flex; flex-wrap: wrap; gap: 8px; }
.chiplink {
  display: inline-flex; align-items: center; min-height: 44px; padding: 6px 14px;
  background: var(--panel); border: 1px solid var(--line); border-radius: 99px;
  color: var(--text); text-decoration: none; font-size: .92rem;
}
.plist { display: grid; gap: 8px; }
@media (min-width: 720px) { .plist { grid-template-columns: 1fr 1fr; } }
.pitem {
  display: flex; align-items: center; gap: 12px; min-height: 64px; padding: 10px 14px;
  background: var(--panel); border: 1px solid var(--line); border-radius: 14px;
  color: var(--text); text-decoration: none;
}
.num {
  flex: none; display: inline-grid; place-items: center; width: 34px; height: 34px;
  border-radius: 10px; background: var(--panel2); font-weight: 700; color: var(--dim); font-size: .9rem;
}
.pitem .pt { flex: 1; min-width: 0; }
.pitem .pt b { display: block; font-weight: 600; }
.pitem .pt small { color: var(--dim); }

/* ---------- problem page ---------- */
.seg {
  position: sticky; top: calc(var(--top) + 61px); z-index: 5;
  display: flex; gap: 4px; padding: 4px; margin: -4px 0 14px;
  background: var(--panel); border: 1px solid var(--line); border-radius: 12px;
}
.seg a {
  flex: 1; display: grid; place-items: center; min-height: 40px; border-radius: 9px;
  color: var(--dim); text-decoration: none; font-size: .88rem; font-weight: 600;
}
.seg a.on { background: var(--panel2); color: var(--text); }
.io { display: grid; gap: 10px; grid-template-columns: 1fr; }
@media (min-width: 720px) { .io { grid-template-columns: 1fr 1fr; } }
.io .label { margin-top: 0; }
.file { margin: 14px 0; }
.file-head { display: flex; justify-content: space-between; align-items: center; gap: 8px; margin-bottom: 6px; }
.file .prose { background: var(--panel); border: 1px solid var(--line); border-radius: 10px; padding: 10px 12px; font-size: .94rem; }
.file .prose + pre { margin-top: 8px; }

.steps { display: grid; gap: 12px; }
.step { display: grid; gap: 8px; grid-template-columns: minmax(0, 1fr); padding-bottom: 12px; border-bottom: 1px solid var(--line); }
@media (min-width: 800px) {
  .step { grid-template-columns: minmax(0, 1.15fr) minmax(0, 1fr); gap: 16px; align-items: start; }
}
.say p { margin: 0 0 6px; }
.py .label { margin: 8px 0 4px; }
pre.code.pyc { background: #0d1210; border-color: #24332b; }
.vs { display: grid; gap: 10px; grid-template-columns: minmax(0, 1fr); }
@media (min-width: 720px) { .vs { grid-template-columns: minmax(0, 1fr) minmax(0, 1fr); } }
.vs .label { margin-top: 0; }
.analogy { background: var(--accent-bg); border-radius: 12px; padding: 12px 14px; margin: 18px 0; }
.tablewrap { overflow-x: auto; border: 1px solid var(--line); border-radius: 10px; }
table.trace { border-collapse: collapse; width: 100%; font: 14px/1.4 var(--mono); }
table.trace th, table.trace td { padding: 8px 12px; border-bottom: 1px solid var(--line); text-align: left; white-space: nowrap; }
table.trace th { background: var(--panel2); color: var(--dim); font-weight: 600; }
table.trace tr:last-child td { border-bottom: 0; }
.mistakes li::marker { content: "⚠ "; }

/* ---------- exercises ---------- */
.sess-head { display: flex; align-items: center; justify-content: space-between; gap: 8px; font-size: .9rem; color: var(--dim); }
.bar { height: 4px; background: var(--panel2); border-radius: 4px; margin: 8px 0 14px; overflow: hidden; }
.bar i { display: block; height: 100%; background: var(--accent); }
.from { font-size: .8rem; color: var(--dim); }
.q { font-size: 1.05rem; font-weight: 600; margin: 4px 0 12px; }
.sess-nav { display: grid; grid-template-columns: 1fr auto 1fr; gap: 8px; margin-top: 18px; padding: 8px 0; }
.sess-nav .btn { margin: 0; }
.sess-nav .btn.primary.pulse { box-shadow: 0 0 0 3px var(--accent-bg); }

.choices { display: grid; gap: 8px; margin: 10px 0; }
.choice {
  display: block; width: 100%; min-height: 52px; padding: 12px 14px; text-align: left;
  background: var(--panel); color: var(--text); border: 1px solid var(--line); border-radius: 12px;
  font: inherit; cursor: pointer;
}
.choice:disabled { cursor: default; }
.choice.right { border-color: var(--ok); background: var(--ok-bg); }
.choice.wrong { border-color: var(--bad); background: var(--bad-bg); }
.cc { display: block; font: 14px/1.5 var(--mono); white-space: pre; overflow-x: auto; }
.cc.out { color: #e9e3c9; }
.ct { display: block; }

.blank {
  display: inline-block; min-width: 3.2em; min-height: 1.7em; margin: 0 2px; padding: 0 6px;
  border: 0; border-bottom: 2px solid var(--accent); border-radius: 4px 4px 0 0;
  background: var(--accent-bg); color: var(--text); font: inherit; vertical-align: baseline; cursor: pointer;
}
.blank.active { outline: 2px solid var(--accent); }
.blank.filled { background: var(--panel2); }
.blank.right { border-color: var(--ok); background: var(--ok-bg); }
.blank.wrong { border-color: var(--bad); background: var(--bad-bg); }
.bank { display: flex; flex-wrap: wrap; gap: 8px; margin: 12px 0; }
.chip {
  min-height: 48px; min-width: 52px; padding: 8px 14px;
  background: var(--panel2); color: var(--text); border: 1px solid var(--line); border-radius: 12px;
  font: 15px var(--mono); cursor: pointer;
}
.chip:disabled { opacity: .5; }
mark { background: var(--ok-bg); color: inherit; border-bottom: 2px solid var(--ok); border-radius: 3px; padding: 0 2px; }

.placed { min-height: 64px; padding: 6px; background: var(--codebg); border: 1px solid var(--line); border-radius: 10px; overflow-x: auto; }
.placed .empty { padding: 14px; color: var(--dim); font-size: .9rem; }
.pool { display: grid; gap: 6px; overflow-x: auto; padding-bottom: 2px; }
.oline, .pline {
  display: block; width: max-content; min-width: 100%; min-height: 44px; padding: 9px 12px; text-align: left;
  font: 14px/1.4 var(--mono); white-space: pre; cursor: pointer;
  color: var(--text); border-radius: 8px;
}
.oline { background: transparent; border: 1px solid transparent; min-height: 40px; }
.oline:not(:disabled):active { background: var(--panel2); }
.oline.wrong { border-color: var(--bad); background: var(--bad-bg); }
.pline { background: var(--panel); border: 1px solid var(--line); }

.buglines { overflow-x: auto; background: var(--codebg); border: 1px solid var(--line); border-radius: 10px; padding: 4px; }
.bl-inner { display: inline-block; min-width: 100%; }
.bugline {
  display: block; width: 100%; min-height: 40px; padding: 8px 10px; text-align: left;
  background: none; border: 1px solid transparent; border-radius: 8px; color: var(--text);
  font: 14px/1.4 var(--mono); white-space: pre; cursor: pointer;
}
.bugline.right { border-color: var(--ok); background: var(--ok-bg); }
.bugline.wrong { border-color: var(--bad); background: var(--bad-bg); }
.steplist { list-style: none; padding: 0; }

textarea.cold, input.typed {
  width: 100%; margin: 10px 0 4px; padding: 12px;
  background: var(--codebg); color: var(--text); border: 1px solid var(--line); border-radius: 10px;
  font: 16px/1.5 var(--mono);
}
.slot { background: var(--accent-bg); color: var(--accent); border-radius: 4px; padding: 0 4px; }
.note { font-size: .85rem; color: var(--warn); }
pre.diff { background: var(--codebg); border: 1px solid var(--line); border-radius: 10px; padding: 8px 0; }
pre.diff span { display: block; padding: 0 12px; }
.dm { background: var(--bad-bg); color: #ffb3b3; }
.dp { background: var(--warn-bg); color: #ffd99a; }

.fb { margin-top: 14px; padding: 12px 14px; border-radius: 12px; border: 1px solid; }
.fb.ok { border-color: var(--ok); background: var(--ok-bg); }
.fb.bad { border-color: var(--bad); background: var(--bad-bg); }
.fb-head { font-weight: 700; font-size: 1.05rem; }
.fb .label { color: var(--text); opacity: .7; }
.fb .whywrong { margin-top: 6px; }

.summary { text-align: center; }
.summary ul { text-align: left; }
.bigscore { font-size: 2.6rem; font-weight: 800; }

/* ---------- reference screens ---------- */
details.rule summary {
  display: flex; align-items: center; gap: 10px; min-height: 44px; list-style: none; cursor: pointer; font-weight: 600;
}
details.rule summary::-webkit-details-marker { display: none; }
details.rule[open] summary { margin-bottom: 8px; }
.missdot { margin-left: auto; width: 9px; height: 9px; border-radius: 50%; background: var(--bad); flex: none; }
.search {
  width: 100%; min-height: 48px; padding: 10px 14px; margin-bottom: 6px;
  background: var(--panel); color: var(--text); border: 1px solid var(--line); border-radius: 12px; font: inherit;
}
pre.msg { font: 600 15px/1.4 var(--mono); color: var(--bad); white-space: pre-wrap; }
.says { font-weight: 600; }
.ptable .prow { display: grid; grid-template-columns: auto 1fr auto; gap: 4px 12px; align-items: center; padding: 10px 0; border-top: 1px solid var(--line); text-decoration: none; color: var(--text); }
.ptable .prow small { grid-column: 2 / 4; color: var(--dim); }