:root {
  color-scheme: light;
  --page: #f7f8f5;
  --paper: #ffffff;
  --ink: #1c1917;
  --muted: #5f5a52;
  --line: #d6d3c8;
  --soft: #fffdf8;
  --teal: #0f6f67;
  --teal-dark: #0b5650;
  --red: #a53a2b;
  --gold: #f0b429;
  --blue: #486fae;
}

* {
  box-sizing: border-box;
}

html {
  background: var(--page);
}

body {
  margin: 0;
  background: var(--page);
  color: var(--ink);
  font-family: Arial, Helvetica, sans-serif;
}

button {
  font: inherit;
}

a {
  color: inherit;
}

.shell {
  min-height: 100vh;
}

.top {
  border-bottom: 1px solid var(--line);
  background: var(--paper);
}

.top-inner,
.layout {
  width: min(100%, 1280px);
  margin: 0 auto;
}

.top-inner {
  padding: 20px 16px 24px;
}

.hero {
  display: flex;
  align-items: flex-end;
  justify-content: space-between;
  gap: 24px;
}

.badge {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  margin-bottom: 14px;
  border: 1px solid #bfe8df;
  border-radius: 6px;
  background: #ecfbf7;
  padding: 6px 10px;
  color: #09433f;
  font-size: 14px;
  font-weight: 700;
}

.badge-icon {
  width: 16px;
  height: 16px;
}

h1 {
  margin: 0;
  font-size: clamp(30px, 5vw, 44px);
  line-height: 1.08;
  letter-spacing: 0;
}

.lede {
  max-width: 860px;
  margin: 12px 0 0;
  color: var(--muted);
  font-size: 16px;
  line-height: 1.7;
}

.stats {
  display: grid;
  grid-template-columns: repeat(2, minmax(92px, 1fr));
  gap: 12px;
  flex: 0 0 auto;
}

.stat {
  border: 1px solid var(--line);
  border-radius: 8px;
  background: var(--soft);
  padding: 14px 16px;
  color: var(--muted);
  font-size: 14px;
}

.stat strong {
  display: block;
  color: var(--ink);
  font-size: 28px;
  line-height: 1;
}

.main-tabs {
  margin-top: 22px;
  overflow-x: auto;
  padding-bottom: 2px;
}

.tab-row {
  display: flex;
  min-width: max-content;
  gap: 8px;
}

.tab,
.subtab,
.action {
  border: 1px solid var(--line);
  border-radius: 6px;
  background: var(--paper);
  color: var(--ink);
  cursor: pointer;
  font-weight: 700;
}

.tab {
  padding: 10px 12px;
  font-size: 14px;
}

.tab[aria-selected="true"] {
  border-color: var(--ink);
  background: var(--ink);
  color: white;
}

.layout {
  display: grid;
  grid-template-columns: 280px minmax(0, 1fr);
  gap: 24px;
  padding: 24px 16px 40px;
}

.sidebar {
  min-width: 0;
}

.sidebar-card {
  position: sticky;
  top: 16px;
  min-width: 0;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: var(--paper);
  padding: 12px;
  box-shadow: 0 1px 4px rgb(28 25 23 / 0.08);
}

.sidebar-title {
  display: flex;
  align-items: center;
  gap: 8px;
  margin: 0 8px 12px;
  font-size: 14px;
  font-weight: 800;
}

.subtabs {
  display: flex;
  flex-direction: column;
  gap: 8px;
}

.subtab {
  width: 100%;
  min-height: 44px;
  padding: 10px 12px;
  background: var(--soft);
  text-align: left;
  font-size: 14px;
  line-height: 1.35;
}

.subtab[aria-selected="true"] {
  border-color: var(--teal);
  background: var(--teal);
  color: white;
}

.content {
  min-width: 0;
}

.intro,
.question,
.source {
  border: 1px solid var(--line);
  border-radius: 8px;
  background: var(--paper);
  box-shadow: 0 1px 4px rgb(28 25 23 / 0.08);
}

.intro {
  padding: 20px;
}

.intro-grid {
  display: grid;
  grid-template-columns: minmax(0, 1fr) minmax(360px, 430px);
  gap: 24px;
  align-items: start;
}

.focus {
  margin: 0 0 8px;
  color: var(--red);
  font-size: 14px;
  font-weight: 800;
}

h2 {
  margin: 0;
  font-size: 28px;
  line-height: 1.18;
  letter-spacing: 0;
}

.explanation {
  margin: 18px 0 0;
  color: #302c27;
  font-size: 16px;
  line-height: 1.8;
}

.formula-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 12px;
  margin-top: 18px;
}

.formula {
  overflow-x: auto;
  border: 1px solid var(--line);
  border-radius: 6px;
  background: var(--soft);
  padding: 12px;
}

.katex-display {
  margin: 0;
}

.diagram {
  width: 100%;
  overflow: hidden;
}

.diagram svg {
  display: block;
  width: 100%;
  height: auto;
}

.questions {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 16px;
  margin-top: 24px;
}

.question {
  padding: 20px;
}

.question-head {
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  gap: 14px;
}

.level {
  margin: 0 0 8px;
  color: var(--teal-dark);
  font-size: 14px;
  font-weight: 800;
}

.prompt {
  margin: 0;
  font-size: 16px;
  line-height: 1.7;
}

.sigma {
  flex: 0 0 auto;
  color: var(--red);
  font-size: 24px;
  font-weight: 800;
}

.steps {
  margin-top: 18px;
}

.step {
  display: flex;
  gap: 12px;
  border-left: 2px solid var(--teal);
  padding: 6px 0 6px 12px;
  color: #3d3831;
  font-size: 14px;
  line-height: 1.7;
}

.step + .step {
  margin-top: 8px;
}

.step-number {
  display: inline-flex;
  flex: 0 0 auto;
  align-items: center;
  justify-content: center;
  width: 24px;
  height: 24px;
  border-radius: 6px;
  background: var(--teal);
  color: white;
  font-size: 12px;
  font-weight: 800;
}

.actions {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  margin-top: 18px;
}

.action {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  min-height: 38px;
  padding: 8px 12px;
  font-size: 14px;
}

.action.primary {
  border-color: var(--ink);
  background: var(--ink);
  color: white;
}

.action.primary:disabled {
  border-color: #d1d5db;
  background: #d1d5db;
  color: #57534e;
  cursor: not-allowed;
}

.source {
  margin-top: 24px;
  padding: 18px 20px;
  color: var(--muted);
  font-size: 14px;
  line-height: 1.7;
}

.source a {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  color: var(--teal-dark);
  font-weight: 800;
  text-underline-offset: 4px;
}

@media (max-width: 980px) {
  .hero {
    align-items: stretch;
    flex-direction: column;
  }

  .layout {
    grid-template-columns: 1fr;
  }

  .sidebar-card {
    position: static;
  }

  .subtabs {
    flex-direction: row;
    overflow-x: auto;
  }

  .subtab {
    min-width: 220px;
  }

  .intro-grid,
  .questions {
    grid-template-columns: 1fr;
  }
}

@media (max-width: 620px) {
  .top-inner,
  .layout {
    padding-left: 12px;
    padding-right: 12px;
  }

  .stats,
  .formula-grid {
    grid-template-columns: 1fr 1fr;
  }

  .intro,
  .question,
  .source {
    padding: 16px;
  }

  h2 {
    font-size: 24px;
  }
}