:root {
  --bg: #0a0e1a;
  --bg-2: #0d1220;
  --panel: #0f1526;
  --panel-2: #101a30;
  --ink: #e7ebf3;
  --ink-dim: #9098ae;
  --ink-faint: #565f78;
  --cyan: #5fd4e8;
  --amber: #e8b95f;
  --green: #7ee0a8;
  --line: rgba(231, 235, 243, 0.09);
  --font-display: "Space Grotesk", system-ui, sans-serif;
  --font-body: "Source Serif 4", Georgia, serif;
  --font-mono: "IBM Plex Mono", ui-monospace, monospace;
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  min-height: 100vh;
  background:
    radial-gradient(circle at 15% 0%, color-mix(in srgb, var(--cyan) 8%, transparent), transparent 32rem),
    var(--bg);
  color: var(--ink);
  font-family: var(--font-body);
  line-height: 1.7;
}

body.has-course-topbar {
  padding-top: 64px;
}

a { color: inherit; }
button { font: inherit; }

.course-shell {
  width: min(920px, calc(100% - 32px));
  margin: 0 auto;
  padding: 54px 0 72px;
}

.lesson-header {
  padding: clamp(28px, 5vw, 56px) 0 36px;
  border-bottom: 1px solid var(--line);
}

.lesson-kicker {
  margin: 0 0 12px;
  color: var(--cyan);
  font-family: var(--font-mono);
  font-size: .78rem;
  font-weight: 600;
  letter-spacing: .12em;
  text-transform: uppercase;
}

.lesson-header h1 {
  max-width: 760px;
  margin: 0;
  font-family: var(--font-display);
  font-size: clamp(2rem, 4.6vw, 3.65rem);
  line-height: 1.04;
  letter-spacing: -.045em;
}

.lesson-summary {
  max-width: 680px;
  margin: 22px 0 0;
  color: var(--ink-dim);
  font-size: clamp(1rem, 2.2vw, 1.18rem);
}

.lesson-section {
  padding: 44px 0;
  border-bottom: 1px solid var(--line);
}

.lesson-section h2 {
  margin: 0 0 18px;
  font-family: var(--font-display);
  font-size: clamp(1.55rem, 3vw, 2.25rem);
  line-height: 1.15;
  letter-spacing: -.025em;
}

.lesson-section h3 {
  margin: 26px 0 10px;
  font-family: var(--font-display);
}

.lesson-section p,
.lesson-section li {
  color: var(--ink-dim);
}

.concept-card,
.interactive-card {
  margin-top: 22px;
  padding: clamp(20px, 4vw, 32px);
  border: 1px solid var(--line);
  border-radius: 18px;
  background: color-mix(in srgb, var(--panel) 94%, transparent);
}

.interactive-card {
  border-color: color-mix(in srgb, var(--cyan) 35%, var(--line));
}

.formula {
  overflow-x: auto;
  padding: 18px;
  border-radius: 12px;
  background: var(--panel-2);
  color: var(--ink);
  font-family: var(--font-mono);
  text-align: center;
}

.worked-example {
  display: grid;
  grid-template-columns: minmax(120px, 180px) 1fr;
  gap: 22px;
  align-items: center;
}

.worked-visual {
  min-height: 130px;
  display: grid;
  place-items: center;
  border: 1px solid var(--line);
  border-radius: 12px;
  background: var(--panel-2);
}

.worked-visual > svg {
  display: block;
  width: 100%;
  height: 100%;
}

.worked-steps .row {
  display: flex;
  justify-content: space-between;
  gap: 18px;
  padding: 9px 0;
  border-bottom: 1px dashed var(--line);
  font-family: var(--font-mono);
  font-size: .9rem;
}

.worked-steps .row:last-child { border-bottom: 0; }
.worked-steps .l { color: var(--ink-dim); }
.worked-steps .r { color: var(--green); }

.legend {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 12px 22px;
  margin-top: 20px;
}

.legend-item { display: flex; gap: 10px; align-items: flex-start; }
.legend-sym {
  min-width: 38px;
  color: var(--cyan);
  font-family: var(--font-mono);
}
.legend-text { color: var(--ink-dim); }

.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;
}

.course-topbar {
  position: fixed;
  z-index: 1200;
  inset: 0 0 auto;
  height: 64px;
  border-bottom: 1px solid var(--line);
  background: color-mix(in srgb, var(--bg) 88%, transparent);
  backdrop-filter: blur(18px);
}

.course-topbar-inner {
  width: min(1180px, calc(100% - 28px));
  height: 100%;
  margin: 0 auto;
  display: flex;
  align-items: center;
  gap: 10px;
}

.course-brand {
  display: inline-flex;
  align-items: center;
  gap: 10px;
  flex: 0 0 auto;
  color: var(--ink);
  font-family: var(--font-display);
  font-weight: 700;
  text-decoration: none;
}

.course-brand-mark {
  width: 36px;
  height: 36px;
  display: grid;
  place-items: center;
  border: 1px solid color-mix(in srgb, var(--cyan) 32%, var(--line));
  border-radius: 11px;
  background: var(--panel);
  color: var(--cyan);
}

.course-brand:hover,
.course-tool-button:hover,
.course-profile-button:hover {
  color: var(--ink);
  border-color: color-mix(in srgb, var(--cyan) 45%, var(--line));
}

.course-tool-button,
.course-profile-button,
.math-ai-close {
  min-height: 36px;
  border: 1px solid var(--line);
  border-radius: 10px;
  background: var(--panel);
  color: var(--ink-dim);
  padding: 0 13px;
  font-family: var(--font-display);
  font-size: .78rem;
  font-weight: 600;
  cursor: pointer;
}

a.course-tool-button {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  text-decoration: none;
}

.course-tool-button span[aria-hidden="true"] {
  min-width: 14px;
  color: var(--cyan);
  font-family: var(--font-mono);
  font-weight: 600;
  text-align: center;
}

.course-search {
  flex: 1 1 360px;
  max-width: 520px;
  margin: 0 auto;
}

.course-search input {
  width: 100%;
  height: 38px;
  border: 1px solid var(--line);
  border-radius: 11px;
  background: var(--panel);
  color: var(--ink);
  padding: 0 14px;
  outline: none;
  font-family: var(--font-display);
}

.course-search input:focus {
  border-color: color-mix(in srgb, var(--cyan) 55%, var(--line));
  box-shadow: 0 0 0 3px color-mix(in srgb, var(--cyan) 10%, transparent);
}

.course-profile-button {
  color: var(--cyan);
}

.math-ai-panel {
  position: fixed;
  z-index: 1300;
  top: 64px;
  right: 0;
  bottom: 0;
  width: min(390px, 100%);
  padding: 20px;
  border-left: 1px solid var(--line);
  background: color-mix(in srgb, var(--bg-2) 96%, transparent);
  box-shadow: -24px 0 60px rgba(0, 0, 0, .28);
  transform: translateX(105%);
  transition: transform .22s ease;
}

.math-ai-panel.is-open { transform: translateX(0); }
.math-ai-head { display: flex; align-items: center; justify-content: space-between; }
.math-ai-head small {
  display: block;
  color: var(--cyan);
  font-family: var(--font-mono);
  font-size: .62rem;
  letter-spacing: .12em;
}
.math-ai-head strong { display: block; font-family: var(--font-display); font-size: 1.25rem; }
.math-ai-close { width: 36px; padding: 0; font-size: 1.25rem; }
.math-ai-body { padding-top: 28px; color: var(--ink-dim); }
.math-ai-placeholder {
  margin-top: 18px;
  padding: 18px;
  border: 1px dashed var(--line);
  border-radius: 12px;
  background: var(--panel);
  font-family: var(--font-display);
  font-size: .85rem;
}

.course-toast {
  position: fixed;
  z-index: 1400;
  left: 50%;
  bottom: 24px;
  padding: 10px 16px;
  border: 1px solid var(--line);
  border-radius: 10px;
  background: var(--panel-2);
  color: var(--ink);
  font-family: var(--font-display);
  font-size: .82rem;
  opacity: 0;
  transform: translate(-50%, 12px);
  pointer-events: none;
  transition: .2s ease;
}
.course-toast.is-visible { opacity: 1; transform: translate(-50%, 0); }

.lesson-course-nav {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 12px;
  margin-top: 42px;
  padding-top: 26px;
  border-top: 1px solid var(--line);
}

.lesson-course-nav a {
  min-height: 72px;
  display: flex;
  flex-direction: column;
  justify-content: center;
  padding: 14px 18px;
  border: 1px solid var(--line);
  border-radius: 12px;
  background: var(--panel);
  color: var(--ink);
  text-decoration: none;
}

.lesson-course-nav a:last-child { text-align: right; }
.lesson-course-nav small {
  color: var(--ink-faint);
  font-family: var(--font-mono);
  font-size: .68rem;
  letter-spacing: .08em;
  text-transform: uppercase;
}
.lesson-course-nav strong {
  margin-top: 3px;
  color: var(--cyan);
  font-family: var(--font-display);
}
.lesson-course-nav a:hover { border-color: color-mix(in srgb, var(--cyan) 40%, var(--line)); }
.lesson-course-nav .nav-disabled {
  opacity: .42;
  pointer-events: none;
}

.lesson-id-warning {
  margin: 0 0 24px;
  padding: 14px 16px;
  border: 1px solid rgba(232, 185, 95, .45);
  border-radius: 12px;
  background: rgba(232, 185, 95, .09);
  color: var(--amber);
  font-family: var(--font-display);
  font-size: .92rem;
}

.answer-reveal {
  margin-top: 18px;
  border: 1px solid color-mix(in srgb, var(--cyan) 24%, var(--line));
  border-radius: 12px;
  background: color-mix(in srgb, var(--panel-2) 84%, transparent);
  overflow: hidden;
}

.answer-reveal summary {
  position: relative;
  list-style: none;
  cursor: pointer;
  padding: 13px 42px 13px 16px;
  color: var(--cyan);
  font-family: var(--font-display);
  font-size: .86rem;
  font-weight: 600;
  user-select: none;
}

.answer-reveal summary::-webkit-details-marker { display: none; }
.answer-reveal summary::after {
  content: "+";
  position: absolute;
  top: 50%;
  right: 16px;
  transform: translateY(-50%);
  color: var(--ink-dim);
  font-family: var(--font-mono);
  font-size: 1.1rem;
}
.answer-reveal[open] summary::after { content: "−"; }
.answer-reveal .answer-content {
  padding: 0 18px 16px;
  border-top: 1px solid var(--line);
  color: var(--ink-dim);
}
.answer-reveal .answer-content ol { margin-bottom: 0; padding-left: 22px; }
.answer-reveal .answer-content li { margin-top: 12px; }

.lesson-footer {
  padding-top: 32px;
  color: var(--ink-faint);
  font-size: .9rem;
  text-align: center;
}

@media (max-width: 680px) {
  .course-shell {
    width: min(100% - 24px, 920px);
    padding-top: 52px;
  }

  .course-topbar-inner { width: calc(100% - 16px); gap: 6px; }
  .course-brand-name,
  .course-notebook-button { display: none; }
  .course-tool-button,
  .course-profile-button { padding: 0 9px; }
  .course-search { min-width: 0; }
  .course-search input { padding: 0 10px; }
  .worked-example,
  .legend,
  .lesson-course-nav { grid-template-columns: 1fr; }
  .lesson-course-nav a:last-child { text-align: left; }
  .lesson-section { padding: 34px 0; }
}