:root {
  --bg: #0f1117;
  --card: #1a1d27;
  --card-hover: #222634;
  --border: #2a2f3d;
  --text: #e6e8ee;
  --muted: #9aa0ad;
  --accent: #5b8cff;
  --accent-soft: rgba(91, 140, 255, 0.12);
  --good: #4ade80;
  --radius: 10px;
  --card-radius: 8px;
  --field-radius: 6px;
  --shadow: 0 1px 2px rgba(0, 0, 0, 0.25);
  --shadow-hover: 0 6px 18px rgba(0, 0, 0, 0.30);
}
@media (prefers-color-scheme: light) {
  :root {
    --bg: #f5f6f9;
    --card: #ffffff;
    --card-hover: #f0f3fb;
    --border: #e2e5ec;
    --text: #1a1d27;
    --muted: #616875;
    --accent: #2f6bff;
    --accent-soft: rgba(47, 107, 255, 0.08);
    --good: #16a34a;
    --shadow: 0 1px 2px rgba(16, 24, 40, 0.06);
    --shadow-hover: 0 8px 24px rgba(16, 24, 40, 0.10);
  }
}
* { box-sizing: border-box; }
body {
  margin: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.5;
  overflow-x: hidden;
  -webkit-text-size-adjust: 100%;
}
.wrap { max-width: 820px; margin: 0 auto; padding: 24px 20px 80px; }
header.site {
  display: flex; align-items: center; gap: 10px;
  padding: 6px 0 26px;
}
header.site .logo {
  font-size: 22px; font-weight: 700; letter-spacing: -0.5px;
  cursor: pointer;
}
header.site .logo span { color: var(--accent); }
.crumbs { font-size: 16.5px; font-weight: 500; color: var(--muted); margin-bottom: 22px; }
.crumbs a { color: var(--accent); text-decoration: none; cursor: pointer; }
.crumbs a:hover { text-decoration: underline; }
h1 { font-size: 26px; margin: 0 0 6px; letter-spacing: -0.5px; }
.sub { color: var(--muted); margin: 0 0 26px; font-size: 15px; }
.grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(230px, 1fr)); gap: 14px; }
.search { position: relative; margin: 0 0 26px; }
.search input {
  width: 100%;
  padding: 14px 16px 14px 44px;
  font-size: 16px;
  background: var(--card);
  border: 1.5px solid var(--border);
  border-radius: 8px;
  color: var(--text);
  outline: none;
  box-shadow: var(--shadow);
  transition: border-color .12s ease, box-shadow .12s ease;
}
.search input:focus { border-color: var(--accent); box-shadow: 0 0 0 3px var(--accent-soft); }
.search input::placeholder { color: var(--muted); }
.search .mag { position: absolute; left: 15px; top: 50%; transform: translateY(-50%); color: var(--muted); font-size: 16px; pointer-events: none; }
.results-head { font-size: 13px; color: var(--muted); margin: 0 0 12px; }
.section-label { display: flex; align-items: center; gap: 7px; font-size: 12.5px; font-weight: 700; letter-spacing: 0.06em; text-transform: uppercase; color: var(--muted); margin: 0 0 13px; }
.section-label.next { margin-top: 30px; }
.featured-grid { grid-template-columns: repeat(auto-fill, minmax(210px, 1fr)); gap: 10px; margin-bottom: 24px; }
.featured-grid .card {
  padding: 12px 14px;
  background: var(--accent-soft);
  border-color: transparent;
  box-shadow: none;
}
.featured-grid .card:hover { transform: translateY(-1px); border-color: var(--accent); box-shadow: var(--shadow); background: var(--accent-soft); }
.featured-grid .card .title { font-size: 14px; font-weight: 600; margin-bottom: 0; }
.featured-grid .card .desc { display: none; }
.featured-grid .card .count { font-size: 11.5px; margin-top: 5px; font-weight: 600; }
.card .count .topic-tag { color: var(--muted); }
.card {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: var(--card-radius);
  padding: 18px;
  cursor: pointer;
  box-shadow: var(--shadow);
  transition: transform .15s ease, background .15s ease, border-color .15s ease, box-shadow .15s ease;
  text-decoration: none;
  color: inherit;
  display: block;
}
.card:hover { background: var(--card-hover); transform: translateY(-3px); border-color: var(--accent); box-shadow: var(--shadow-hover); }
.card .icon { font-size: 26px; margin-bottom: 8px; }
.card .title { font-weight: 650; font-size: 18.5px; margin-bottom: 4px; }
.card .desc { color: var(--muted); font-size: 14px; }
.card .count { color: var(--accent); font-size: 15px; margin-top: 10px; font-weight: 600; }

.formula-box {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 24px;
  margin-bottom: 18px;
  box-shadow: var(--shadow);
}
.eq {
  font-family: "SFMono-Regular", ui-monospace, Menlo, Consolas, monospace;
  background: var(--accent-soft);
  color: var(--text);
  padding: 12px 14px;
  border-radius: 8px;
  font-size: 15px;
  margin: 4px 0 22px;
  overflow-x: auto;
}
.field { margin-bottom: 16px; }
.field label { display: block; font-size: 14px; font-weight: 600; margin-bottom: 6px; }
.field .hint { color: var(--muted); font-weight: 400; font-size: 12.5px; }
.field input {
  width: 100%;
  padding: 13px 14px;
  font-size: 15px;
  background: var(--bg);
  border: 1.5px solid var(--border);
  border-radius: var(--field-radius);
  color: var(--text);
  outline: none;
  transition: border-color .12s ease, box-shadow .12s ease;
}
.field input:hover { border-color: var(--muted); }
.field input:focus { border-color: var(--accent); box-shadow: 0 0 0 3px var(--accent-soft); }
.field input::placeholder { color: var(--muted); opacity: .7; }
.field .unit { color: var(--muted); font-size: 12.5px; margin-left: 4px; }
button.calc {
  width: 100%;
  padding: 14px;
  font-size: 18px;
  font-weight: 650;
  background: var(--accent);
  color: #fff;
  border: none;
  border-radius: 8px;
  cursor: pointer;
  margin-top: 8px;
  box-shadow: var(--shadow);
  transition: filter .12s ease, transform .06s ease;
}
button.calc:hover { filter: brightness(1.08); }
button.calc:active { transform: translateY(1px); }
.result {
  margin-top: 20px;
  padding: 18px;
  border-radius: 8px;
  background: var(--accent-soft);
  border: 1px solid var(--border);
  display: none;
}
.result.show { display: block; }
.result .label { color: var(--muted); font-size: 13px; margin-bottom: 4px; }
.result .value { font-size: 28px; font-weight: 700; color: var(--good); letter-spacing: -0.5px; }
.result .value .u { font-size: 16px; color: var(--muted); font-weight: 500; }
.result.error .value { color: #f87171; font-size: 16px; font-weight: 500; }
.notes { color: var(--muted); font-size: 13px; margin-top: 14px; }
.sliders { margin: 4px 0 6px; padding: 16px; background: var(--bg); border: 1px solid var(--border); border-radius: 8px; }
.sliders-label { font-size: 12px; font-weight: 700; letter-spacing: 0.05em; text-transform: uppercase; color: var(--muted); margin-bottom: 13px; }
.slider { margin-bottom: 15px; }
.slider:last-child { margin-bottom: 0; }
.slider-head { display: flex; justify-content: space-between; align-items: baseline; font-size: 13.5px; font-weight: 600; margin-bottom: 7px; }
.slider-val { color: var(--accent); font-variant-numeric: tabular-nums; }
.slider input[type=range] { width: 100%; accent-color: var(--accent); cursor: pointer; }
.chart-wrap { margin-top: 22px; }
.chart-title { font-size: 13px; font-weight: 600; color: var(--muted); margin-bottom: 10px; }
.chart { width: 100%; max-width: 500px; height: auto; display: block; overflow: visible; }
.chart .grid { stroke: var(--border); stroke-width: 1; }
.chart .area { fill: var(--accent-soft); stroke: none; }
.chart .cline { fill: none; stroke: var(--accent); stroke-width: 2.5; stroke-linejoin: round; stroke-linecap: round; }
.chart .dot { fill: var(--accent); }
.chart .ylab, .chart .xlab { fill: var(--muted); font-size: 11px; }
.chart .axlab { fill: var(--muted); font-size: 12px; font-weight: 600; }
footer { text-align: center; color: var(--muted); font-size: 12.5px; margin-top: 40px; }
@media (max-width: 560px) {
  .wrap { padding: 16px 14px 56px; }
  header.site { padding: 4px 0 20px; }
  h1 { font-size: 22px; }
  .sub { font-size: 14px; }
  .crumbs { font-size: 15px; margin-bottom: 18px; }
  .formula-box { padding: 18px 15px; }
  .eq { font-size: 13px; padding: 11px 12px; }
  .grid, .featured-grid { grid-template-columns: 1fr; }
  .field input, .search input { font-size: 16px; }   /* 16px avoids iOS focus-zoom */
  button.calc { font-size: 17px; padding: 15px; }
  .slider input[type=range] { height: 28px; }
  .slider-head { font-size: 14px; }
  .result .value { font-size: 25px; }
  .chart { max-width: 100%; }
  .chart .ylab, .chart .xlab { font-size: 13px; }
  .chart .axlab { font-size: 13px; }
}