:root {
  --bg: #F3F6FA;
  --surface: #FFFFFF;
  --surface2: #F7F9FC;
  --line: #DCE3EC;
  --text: #17202C;
  --muted: #5B6676;
  --brand: #0F4C81;
  --brand-soft: #E3EEF8;
  --acid: #D1495B;
  --acid-soft: #FBE4E7;
  --base: #2E86AB;
  --base-soft: #E0F0F7;
  --ok: #2E8B57;
  --warn: #B26A00;
  --warn-soft: #FFF3DC;
  --glass: #7D8B9C;
  --bench: #EAF0F6;
  --shadow: 0 1px 2px rgba(16, 24, 40, .06), 0 4px 14px rgba(16, 24, 40, .06);
  --radius: 14px;
  --on-brand: #FFFFFF;
  color-scheme: light;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #0E141C; --surface: #161E29; --surface2: #1B2533; --line: #2A3647; --text: #E7EDF5; --muted: #9AA8BA;
    --brand: #7FB6E6; --brand-soft: #1B2B3E; --acid: #F07A89; --acid-soft: #3A1F25; --base: #6DB8DA; --base-soft: #17303D;
    --ok: #5CC28C; --warn: #F0B04C; --warn-soft: #3A2E17; --glass: #9FB0C3; --bench: #1A2330;
    --shadow: 0 1px 2px rgba(0, 0, 0, .4); --on-brand: #0E141C; color-scheme: dark;
  }
}
:root[data-theme="dark"] {
  --bg: #0E141C; --surface: #161E29; --surface2: #1B2533; --line: #2A3647; --text: #E7EDF5; --muted: #9AA8BA;
  --brand: #7FB6E6; --brand-soft: #1B2B3E; --acid: #F07A89; --acid-soft: #3A1F25; --base: #6DB8DA; --base-soft: #17303D;
  --ok: #5CC28C; --warn: #F0B04C; --warn-soft: #3A2E17; --glass: #9FB0C3; --bench: #1A2330;
  --shadow: 0 1px 2px rgba(0, 0, 0, .4); --on-brand: #0E141C; color-scheme: dark;
}

* { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body {
  margin: 0; background: var(--bg); color: var(--text);
  font: 16px/1.55 system-ui, -apple-system, "Segoe UI", Roboto, "Noto Sans", Arial, sans-serif;
}
a { color: var(--brand); }
sub, sup { font-size: 70%; line-height: 0; }
h1, h2, h3 { line-height: 1.25; margin: 0; }
button { font: inherit; color: inherit; }

/* ---------- header ---------- */
.topbar {
  position: sticky; top: env(safe-area-inset-top, 0px); z-index: 20; display: flex; align-items: center; gap: 12px;
  padding: 10px 16px; background: color-mix(in srgb, var(--surface) 92%, transparent);
  backdrop-filter: blur(8px); border-bottom: 1px solid var(--line);
}
.topbar .logo { display: flex; align-items: center; gap: 10px; text-decoration: none; color: var(--text); font-weight: 700; }
.topbar .logo svg { width: 30px; height: 30px; flex: none; }
.topbar .logo small { display: block; font-weight: 500; color: var(--muted); font-size: 12px; }
.topbar .spacer { flex: 1; }
.topbar .theme { border: 1px solid var(--line); background: var(--surface); border-radius: 999px; padding: 6px 12px; cursor: pointer; font-size: 13px; white-space: nowrap; flex: none; }

main { max-width: 1240px; margin: 0 auto; padding: 18px 16px 60px; }

/* ---------- home ---------- */
.hero { display: grid; grid-template-columns: 1.2fr 1fr; gap: 24px; align-items: center; padding: 18px 0 8px; }
.hero h1 { font-size: clamp(26px, 4vw, 40px); letter-spacing: -0.02em; }
.hero p { color: var(--muted); font-size: 17px; max-width: 60ch; }
.hero .key { background: var(--surface); border: 1px solid var(--line); border-radius: var(--radius); padding: 16px; box-shadow: var(--shadow); }
.hero .key h3 { font-size: 15px; margin-bottom: 8px; }
.hero canvas { width: 100%; height: 170px; display: block; border-radius: 10px; }
.grid-title { font-size: 18px; margin: 26px 0 12px; }
.grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(270px, 1fr)); gap: 14px; }
.exp-card {
  display: flex; flex-direction: column; gap: 8px; text-decoration: none; color: var(--text);
  background: var(--surface); border: 1px solid var(--line); border-radius: var(--radius); padding: 14px; box-shadow: var(--shadow);
  transition: transform .15s ease, border-color .15s ease;
}
.exp-card:hover, .exp-card:focus-visible { transform: translateY(-2px); border-color: var(--brand); outline: none; }
.exp-card .thumb { height: 92px; border-radius: 10px; background: var(--bench); display: grid; place-items: center; }
.exp-card .thumb svg { height: 80px; }
.exp-card .num { font-size: 12px; font-weight: 700; color: var(--brand); letter-spacing: .04em; text-transform: uppercase; }
.exp-card h2 { font-size: 17px; }
.exp-card p { margin: 0; font-size: 14px; color: var(--muted); }
.tags { display: flex; flex-wrap: wrap; gap: 6px; }
.tag { font-size: 11.5px; font-weight: 600; padding: 2px 8px; border-radius: 999px; background: var(--brand-soft); color: var(--brand); }

/* ---------- experiment page ---------- */
.exp-head { margin-bottom: 14px; }
.exp-head .back { display: inline-block; font-size: 14px; text-decoration: none; margin-bottom: 6px; }
.exp-head h1 { font-size: clamp(22px, 3.2vw, 32px); letter-spacing: -0.01em; }
.exp-head .lead { margin: 6px 0 8px; color: var(--muted); max-width: 80ch; }
.exp-grid { display: grid; grid-template-columns: minmax(0, 1.35fr) minmax(0, 1fr); gap: 16px; align-items: start; }
.col-main, .col-side { display: flex; flex-direction: column; gap: 16px; min-width: 0; }
.card { background: var(--surface); border: 1px solid var(--line); border-radius: var(--radius); padding: 14px; box-shadow: var(--shadow); min-width: 0; }
.card h3 { font-size: 15.5px; margin-bottom: 10px; }
.card h4 { font-size: 14px; margin: 12px 0 4px; }
.card p { margin: 6px 0; }
.card ul, .card ol { margin: 6px 0; padding-left: 20px; }
.bench { padding: 0; overflow: hidden; }
.bench .stage { background: var(--bench); position: relative; }
.bench svg.apparatus { display: block; width: 100%; height: auto; max-height: 420px; }
.controls { display: flex; flex-direction: column; gap: 10px; padding: 12px 14px 14px; border-top: 1px solid var(--line); }
.row { display: flex; flex-wrap: wrap; gap: 8px; align-items: center; }
.row > .lbl { font-size: 13px; font-weight: 600; color: var(--muted); min-width: 92px; }

.choice { display: inline-flex; flex-wrap: wrap; gap: 6px; }
.choice button, .btn {
  border: 1px solid var(--line); background: var(--surface2); border-radius: 10px; padding: 7px 12px;
  cursor: pointer; font-size: 14px; line-height: 1.2; min-height: 36px; transition: background .12s, border-color .12s;
}
.choice button:hover, .btn:hover { border-color: var(--brand); }
.choice button.on { background: var(--brand); color: var(--on-brand); border-color: var(--brand); }
.choice button.off { border-color: var(--acid); }
.btn.primary { background: var(--brand); border-color: var(--brand); color: var(--on-brand); font-weight: 600; }
.btn.acid { background: var(--acid); border-color: var(--acid); color: var(--on-brand); font-weight: 600; }
.btn.base { background: var(--base); border-color: var(--base); color: var(--on-brand); font-weight: 600; }
.btn.ghost { background: transparent; }
.btn:disabled, .choice button:disabled { opacity: .45; cursor: not-allowed; }
input[type=range] { width: 100%; accent-color: var(--brand); }

.readouts { display: flex; flex-wrap: wrap; gap: 8px; }
.readout { background: var(--surface2); border: 1px solid var(--line); border-radius: 10px; padding: 6px 10px; min-width: 96px; }
.readout span { display: block; font-size: 11.5px; color: var(--muted); font-weight: 600; text-transform: uppercase; letter-spacing: .04em; }
.readout b { font-size: 17px; font-variant-numeric: tabular-nums; }

.zoom-canvas { width: 100%; display: block; border-radius: 10px; border: 1px solid var(--line); background: #EEF6FC; touch-action: pan-y; }
.legend { display: flex; flex-wrap: wrap; gap: 6px 14px; margin-top: 8px; font-size: 12.5px; color: var(--muted); }
.legend span { display: inline-flex; align-items: center; gap: 6px; }
.legend i { width: 12px; height: 12px; border-radius: 50%; display: inline-block; }
.note { font-size: 13px; color: var(--muted); }

.eq { background: var(--surface2); border: 1px solid var(--line); border-radius: 10px; padding: 8px 12px; margin: 6px 0; font-size: 15.5px; overflow-x: auto; white-space: nowrap; }
.eq.hl { border-color: var(--acid); background: var(--acid-soft); }
.eq.hb { border-color: var(--base); background: var(--base-soft); }
.callout { border-left: 4px solid var(--brand); background: var(--brand-soft); padding: 8px 12px; border-radius: 0 10px 10px 0; margin: 8px 0; font-size: 14.5px; }
.callout.warn { border-color: var(--warn); background: var(--warn-soft); }
.callout.acid { border-color: var(--acid); background: var(--acid-soft); }

.log { list-style: none; margin: 0; padding: 0; display: flex; flex-direction: column; gap: 6px; max-height: 260px; overflow: auto; }
.log li { font-size: 14px; padding: 6px 10px; border-radius: 8px; background: var(--surface2); border: 1px solid var(--line); animation: pop .25s ease; }
.log li.good { border-color: var(--ok); }
.log li.acid { border-color: var(--acid); }
.log li.base { border-color: var(--base); }
.log li.warn { border-color: var(--warn); background: var(--warn-soft); }
.log:empty::before { content: "Your observations will appear here."; color: var(--muted); font-size: 14px; }
@keyframes pop { from { transform: translateY(-4px); opacity: 0; } }

table.results { width: 100%; border-collapse: collapse; font-size: 13px; }
table.results th, table.results td { border: 1px solid var(--line); padding: 5px 6px; text-align: center; }
table.results th { background: var(--surface2); font-weight: 600; }
table.results td.sw { font-size: 11.5px; }
table.results td .dot { display: inline-block; width: 14px; height: 14px; border-radius: 50%; border: 1px solid rgba(0,0,0,.15); vertical-align: -3px; margin-right: 4px; }
.table-wrap { overflow-x: auto; }

.steps { counter-reset: s; list-style: none; padding: 0 !important; }
.steps li { counter-increment: s; position: relative; padding-left: 30px; margin: 6px 0; font-size: 14.5px; }
.steps li::before { content: counter(s); position: absolute; left: 0; top: 1px; width: 21px; height: 21px; border-radius: 50%; background: var(--brand-soft); color: var(--brand); font-size: 12px; font-weight: 700; display: grid; place-items: center; }
.steps li.done::before { background: var(--ok); color: var(--on-brand); content: "\2713"; }
.steps li.now { font-weight: 600; }

.pager { display: flex; justify-content: space-between; gap: 12px; margin-top: 18px; }
.pager a { flex: 1; text-decoration: none; background: var(--surface); border: 1px solid var(--line); border-radius: var(--radius); padding: 10px 14px; color: var(--text); }
.pager a small { display: block; color: var(--muted); font-size: 12px; }
.pager a.next { text-align: right; }

/* ---------- SVG apparatus ---------- */
.glass { fill: rgba(255, 255, 255, .35); stroke: var(--glass); stroke-width: 2.5; stroke-linejoin: round; }
.liquid { transition: fill .6s ease; }
.tube-outer { fill: none; stroke: var(--glass); stroke-width: 9; stroke-linecap: round; stroke-linejoin: round; }
.tube-inner { fill: none; stroke: #F4F8FB; stroke-width: 5; stroke-linecap: round; stroke-linejoin: round; }
.svg-label { font: 600 13px system-ui, sans-serif; fill: var(--text); }
.svg-label.small { font-size: 11.5px; fill: var(--muted); font-weight: 500; }
.svg-label.big { font-size: 16px; font-weight: 700; }
.stand { fill: #9AA6B4; }
.wood { fill: #C79A6B; }
.metal { fill: #9EA7B3; }

.footer { text-align: center; color: var(--muted); font-size: 13px; padding: 30px 16px; }

@media (max-width: 900px) {
  .exp-grid, .hero { grid-template-columns: minmax(0, 1fr); }
}
@media (max-width: 520px) {
  main { padding: 12px 12px 40px; }
  .row > .lbl { min-width: 100%; }
  .card { padding: 12px; }
  .btn, .choice button { padding: 7px 10px; font-size: 13.5px; }
}
@media (prefers-reduced-motion: reduce) {
  .exp-card, .log li { transition: none; animation: none; }
}