:root {
  color-scheme: dark;
  --bg: #10111a;
  --panel: #191c2a;
  --plot: #111320;
  --border: #363a50;
  --text: #f6f4fa;
  --muted: #c0c2d7;
  --lime: #dcff85;
  --violet: #c8b7ff;
  --orange: #ffac82;
  --mono: Consolas, 'Courier New', monospace;
  font-family: Arial, 'Segoe UI', sans-serif;
  background: var(--bg);
  color: var(--text);
}
*, *::before, *::after { box-sizing: border-box; }
body { margin: 0; }
[hidden] { display: none !important; }
a { color: inherit; text-decoration: none; }
button, input { font: inherit; }
button, summary { cursor: pointer; }
:focus-visible { outline: 2px solid var(--lime); outline-offset: 3px; }
.skip { position: fixed; top: 8px; right: 8px; transform: translateY(-150%); background: var(--lime); color: var(--bg); z-index: 3; padding: 10px; }
.skip:focus { transform: none; }
.wrap { width: min(1220px, calc(100% - 48px)); margin-inline: auto; }
.topbar { border-bottom: 1px solid var(--border); }
.bar-inner, .brand, nav { display: flex; align-items: center; }
.bar-inner { justify-content: space-between; min-height: 62px; }
.brand { gap: 9px; font-weight: 750; }
.brand-icon { display: grid; place-items: center; width: 34px; height: 34px; border: 1px solid #56633e; border-radius: 9px; background: #252c20; color: var(--lime); font: 24px var(--mono); }
nav { gap: clamp(12px, 3.5vw, 32px); color: var(--muted); font-size: .91rem; }
nav a:hover, footer a:hover { color: var(--lime); }
h1, h2, h3, h4, p { margin: 0; }
p { line-height: 1.65; }
.math { font-family: var(--mono); font-weight: 500; direction: ltr; unicode-bidi: isolate; }
.hero { padding: 40px 0 28px; border-bottom: 1px solid var(--border); }
.eyebrow { color: var(--lime); font-size: .79rem; font-weight: 700; letter-spacing: .04em; }
.eyebrow span { display: inline-block; width: 7px; height: 7px; margin-inline-end: 4px; border-radius: 50%; background: var(--lime); vertical-align: 1px; }
h1 { font-size: clamp(2.75rem, 7.1vw, 5.5rem); letter-spacing: -.07em; line-height: .99; margin: 10px 0 16px; }
.hero-copy { font-size: clamp(1.05rem, 2vw, 1.28rem); color: var(--muted); max-width: 56ch; }
.panel { background: var(--panel); border: 1px solid var(--border); border-radius: 14px; }
.idea-grid { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: 10px; margin-top: 21px; }
.idea-grid article { padding: 14px 16px; }
.idea-grid h2 { font-size: 1.17rem; margin: 4px 0 5px; }
.idea-grid p:not(.eyebrow), .definition p, .section-head > p, .checklist p, .tip p { color: var(--muted); }
.hero-math { display: block; color: var(--lime); margin-top: 6px; text-align: right; }
.definition { margin-top: 10px; padding: 11px 16px; }
.definition summary, .step summary { font-weight: 700; }
.definition p { margin-top: 8px; }
.equations { display: flex; flex-wrap: wrap; gap: 6px; margin-top: 8px; }
.equations bdi { padding: 6px 9px; background: var(--plot); border-radius: 6px; color: var(--lime); font-size: .88rem; }
section { scroll-margin-top: 20px; }
#lab, .lesson { padding-top: 28px; }
.section-head { display: flex; align-items: end; justify-content: space-between; flex-wrap: wrap; gap: 6px 20px; margin-bottom: 13px; }
.section-head h2 { font-size: clamp(1.6rem, 4vw, 2.4rem); margin-top: 3px; }
.examples { display: grid; grid-template-columns: repeat(4, minmax(0, 1fr)); gap: 8px; margin-bottom: 12px; }
.examples button, .segments button, .point-options button, .secondary {
  border: 1px solid var(--border); background: var(--panel); color: var(--text);
  border-radius: 8px; padding: 7px 10px; min-height: 36px;
}
.examples button { text-align: right; font-weight: 700; padding: 9px 12px; }
.examples button span { display: block; color: var(--muted); font: .82rem var(--mono); margin-bottom: 4px; }
button[aria-pressed='true'] { color: var(--lime); background: #272f22; border-color: #84945f; }
button[aria-pressed='true'] span { color: var(--lime); }
button:hover { border-color: var(--lime); }
.exercise-grid { display: grid; grid-template-columns: minmax(0, 1fr) minmax(0, 1.23fr); align-items: start; gap: 12px; }
.exercise, .lab { padding: 16px; min-width: 0; }
.exercise h3 { font-size: clamp(1.18rem, 2.3vw, 1.65rem); margin: 5px 0 10px; }
#question { color: var(--muted); }
.solution-header, .plot-header { display: flex; gap: 8px 12px; justify-content: space-between; align-items: center; flex-wrap: wrap; }
.solution-header { margin-top: 15px; border-top: 1px solid var(--border); padding-top: 12px; }
.secondary { background: #25293b; font-size: .83rem; }
.steps { display: grid; gap: 7px; margin-top: 8px; }
.step { border: 1px solid var(--border); border-radius: 8px; background: #171924; }
.step summary { padding: 10px; font-size: .91rem; }
.step summary::marker { color: var(--lime); }
.step-number { color: var(--lime); font: .79rem var(--mono); margin-inline: 4px 6px; }
.step-body { padding: 2px 11px 10px; color: var(--muted); font-size: .94rem; line-height: 1.6; }
.step-body strong { color: var(--text); }
.formula {
  direction: ltr; text-align: left; background: var(--plot); border-radius: 7px;
  color: var(--text); padding: 7px 9px; margin-block: 6px; font-size: clamp(.76rem, 1.2vw, .96rem);
  overflow-wrap: anywhere;
}
.cases { display: flex; align-items: center; gap: 7px; flex-wrap: wrap; }
.cases > div { line-height: 1.8; }
.case-brace { width: 6px; height: 34px; border: solid var(--muted); border-width: 1px 0 1px 1px; border-radius: 5px 0 0 5px; }
.case-condition { color: var(--muted); margin-inline-start: 8px; white-space: nowrap; }
.lab { background: #171a29; }
.plot-header h3 { font-size: 1.18rem; margin-top: 2px; }
.stage { border: 1px solid var(--border); border-radius: 10px; background: radial-gradient(ellipse at 55% 44%, #20243b, var(--plot) 76%); margin-top: 10px; }
#graph { display: block; width: 100%; height: clamp(285px, 38vw, 420px); touch-action: none; cursor: grab; border-radius: 9px; }
#graph:active { cursor: grabbing; }
#plot-help { color: var(--muted); font-size: .77rem; text-align: center; padding: 1px 7px 8px; }
.legend { list-style: none; display: flex; align-items: center; flex-wrap: wrap; gap: 5px 15px; padding: 0; margin: 8px 0; color: var(--muted); font-size: .78rem; }
.legend li { display: flex; align-items: center; gap: 5px; }
.legend i { display: inline-block; width: 16px; border-top: 2px solid; }
.legend .surface-key { color: var(--violet); }
.legend .plane-key { color: var(--lime); border-top-style: dashed; }
.legend .error-key { color: var(--orange); }
.legend .path-key { color: #8edbea; border-top-style: dotted; }
.plot-insight { padding: 8px 10px; background: #23293a; border-radius: 8px; color: var(--muted); font-size: .88rem; line-height: 1.55; }
.controls { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 12px; margin-top: 11px; }
.controls label { display: flex; justify-content: space-between; flex-wrap: wrap; gap: 2px 6px; font-size: .85rem; font-weight: 700; }
output { color: var(--lime); font: .8rem var(--mono); }
input[type='range'] { display: block; width: 100%; margin: 6px 0 0; accent-color: var(--lime); cursor: pointer; }
.plot-actions, .point-options, .segments { display: flex; flex-wrap: wrap; gap: 5px; align-items: center; }
.plot-actions { margin-top: 9px; gap: 7px 10px; }
.point-options { margin-top: 8px; }
.point-options button, .segments button { min-height: 32px; padding: 4px 8px; font-size: .78rem; }
.zoom { display: flex; align-items: center; gap: 5px; font-size: .82rem; cursor: pointer; }
.zoom input { accent-color: var(--lime); width: 16px; height: 16px; margin: 0; }
.coordinates { color: var(--muted); font-size: .83rem; margin-top: 8px; }
.metrics { display: grid; grid-template-columns: repeat(4, minmax(0, 1fr)); gap: 5px; margin-top: 6px; }
.metric { background: var(--panel); border: 1px solid var(--border); border-radius: 8px; padding: 6px 8px; display: grid; gap: 5px; }
.metric span { color: var(--muted); font-size: .75rem; }
.metric > bdi { justify-self: start; font-size: clamp(.83rem, 1.4vw, 1.1rem); overflow-wrap: anywhere; }
.metric.focus { border-color: #829756; background: #252e20; }
.metric.focus > bdi { color: var(--lime); }
.checklist { list-style: none; margin: 0; padding: 0; display: grid; gap: 8px; }
.checklist li { display: flex; align-items: baseline; gap: 12px; padding: 13px 15px; }
.checklist h3 { font-size: 1.05rem; margin-bottom: 3px; }
.checklist li > span { color: var(--lime); font: .85rem var(--mono); }
.tip { margin-top: 10px; padding: 13px 15px; border-color: #575273; }
.tip strong { color: var(--violet); }
footer { display: flex; gap: 8px; margin-top: 30px !important; padding-block: 16px; border-top: 1px solid var(--border); color: var(--muted); font-size: .82rem; }
footer a { margin-inline-start: auto; }
@media (max-width: 850px) {
  .wrap { width: calc(100% - 30px); }
  .exercise-grid { grid-template-columns: minmax(0, 1fr); }
  .idea-grid { grid-template-columns: minmax(0, 1fr); }
  #graph { height: clamp(290px, 59vw, 420px); }
}
@media (max-width: 520px) {
  .examples, .metrics { grid-template-columns: repeat(2, minmax(0, 1fr)); }
  .hero { padding-top: 26px; }
  .exercise, .lab { padding: 12px; }
  .equations bdi { font-size: .75rem; overflow-wrap: anywhere; }
}