:root {
  --ink: #1c1914;
  --ink-soft: #5a5248;
  --paper: #f4efe4;
  --card: #fffdf8;
  --accent: #7a4a2a;
  --accent-soft: #efe0d0;
  --good: #2b6b45;
  --bad: #a33a2c;
  --trap: #2b241c;
  --trap-ink: #f4efe4;
  --trap-gold: #e6b17a;
  --border: #d8c8b0;
  --radius: 14px;
  font-size: 17px;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  font-family: "Iowan Old Style", "Palatino Linotype", Palatino, Georgia, serif;
  background: var(--paper);
  color: var(--ink);
  line-height: 1.5;
}

.kicker {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  font-size: 0.78rem;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  color: var(--accent);
  font-weight: 700;
  margin: 0 0 0.4rem;
}

header.site-header {
  padding: 2rem 1.5rem 1.5rem;
  border-bottom: 1px solid var(--border);
  background: var(--card);
}

header.site-header h1 {
  margin: 0 0 0.4rem;
  font-size: 1.75rem;
  line-height: 1.15;
}

header.site-header p:last-child {
  margin: 0;
  color: var(--ink-soft);
  max-width: 40rem;
}

main {
  max-width: 820px;
  margin: 0 auto;
  padding: 1.5rem 1.15rem 3rem;
}

a.back-link {
  display: inline-block;
  margin-bottom: 1rem;
  color: var(--accent);
  text-decoration: none;
  font-weight: 700;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
}
a.back-link:hover { text-decoration: underline; }

.card-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(260px, 1fr));
  gap: 1rem;
}

.topic-card {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1.2rem 1.2rem 1.15rem;
  display: flex;
  flex-direction: column;
  gap: 0.45rem;
}

.topic-card h2 {
  margin: 0;
  font-size: 1.2rem;
  line-height: 1.2;
}

.topic-card .meta {
  color: var(--ink-soft);
  font-size: 0.92rem;
}

.badge {
  align-self: flex-start;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  font-size: 0.72rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.04em;
  padding: 0.18rem 0.55rem;
  border-radius: 999px;
  background: var(--accent-soft);
  color: var(--accent);
}

.topic-card .study-btn {
  margin-top: 0.55rem;
  display: inline-block;
  text-align: center;
  background: var(--accent);
  color: #fff;
  text-decoration: none;
  padding: 0.6rem 1rem;
  border-radius: 10px;
  font-weight: 700;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
}
.topic-card .study-btn:hover { opacity: 0.92; }

.study-header h1 { margin: 0 0 0.2rem; font-size: 1.7rem; line-height: 1.15; }
.study-header .meta { color: var(--ink-soft); margin: 0 0 1.25rem; }

section.block {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1.15rem 1.25rem 1.2rem;
  margin-bottom: 1.15rem;
}

section.block h2 {
  margin: 0 0 0.75rem;
  font-size: 1.15rem;
  border-bottom: 1px solid var(--border);
  padding-bottom: 0.45rem;
}

.summary { margin: 0; color: var(--ink); }

.formula-list { display: flex; flex-direction: column; gap: 0.85rem; }
.formula {
  padding: 0.7rem 0.85rem;
  background: var(--paper);
  border-radius: 10px;
}
.formula .name {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  font-size: 0.72rem;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--accent);
  font-weight: 700;
  margin: 0 0 0.3rem;
}
.formula .tex { font-size: 1.15rem; overflow-x: auto; }
.formula .note { margin: 0.35rem 0 0; color: var(--ink-soft); font-size: 0.95rem; }

.method-step { margin-bottom: 1rem; }
.method-step:last-child { margin-bottom: 0; }
.method-step h3 { margin: 0 0 0.3rem; font-size: 1.05rem; }
.method-step p { margin: 0 0 0.35rem; }
.method-step .tex { overflow-x: auto; }

.trap-box {
  background: var(--trap);
  color: var(--trap-ink);
  border-radius: var(--radius);
  padding: 1.1rem 1.2rem;
  margin-bottom: 1.15rem;
}
.trap-box h2 {
  color: var(--trap-gold);
  border-bottom-color: #4a3f34;
}
.trap { margin-bottom: 0.85rem; }
.trap:last-child { margin-bottom: 0; }
.trap h3 { margin: 0 0 0.25rem; font-size: 1.05rem; color: #fff; }
.trap p { margin: 0; color: #e8ddd0; }

.example { margin-bottom: 1.25rem; }
.example:last-child { margin-bottom: 0; }
.example h3 { margin: 0 0 0.4rem; font-size: 1.05rem; }
.given { list-style: none; padding: 0; margin: 0 0 0.5rem; }
.given li { margin-bottom: 0.2rem; }
.ask { margin: 0 0 0.5rem; }
.steps { margin: 0.4rem 0 0; padding-left: 1.2rem; }
.steps li { margin-bottom: 0.45rem; }
.result {
  margin: 0.65rem 0 0;
  padding: 0.55rem 0.75rem;
  background: var(--accent-soft);
  border-radius: 8px;
  font-weight: 700;
}

.tool-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 0.65rem 0.85rem;
  margin-bottom: 0.85rem;
}
.tool-grid label {
  display: flex;
  flex-direction: column;
  gap: 0.25rem;
  font-size: 0.92rem;
  color: var(--ink-soft);
}
.tool-grid input {
  font: inherit;
  padding: 0.45rem 0.55rem;
  border: 1px solid var(--border);
  border-radius: 8px;
  background: #fff;
  color: var(--ink);
}
.tool-actions { margin: 0.4rem 0 0.9rem; }
.tool-out {
  background: var(--paper);
  border-radius: 10px;
  padding: 0.85rem 0.95rem;
}
.tool-out .line { margin: 0 0 0.45rem; overflow-x: auto; }
.tool-out .line:last-child { margin-bottom: 0; }
.tool-error { color: var(--bad); font-weight: 700; }

@media (max-width: 560px) {
  .tool-grid { grid-template-columns: 1fr; }
}

button {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  font-size: 0.95rem;
  cursor: pointer;
  border: 1px solid var(--border);
  background: var(--card);
  color: var(--ink);
  padding: 0.5rem 1rem;
  border-radius: 10px;
}
button.primary { background: var(--accent); color: #fff; border-color: var(--accent); }
button:hover { filter: brightness(0.97); }
button:disabled { opacity: 0.5; cursor: default; }

.flashcard-wrap {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 0.85rem;
}

.flashcard {
  width: 100%;
  max-width: 520px;
  min-height: 160px;
  perspective: 1200px;
  cursor: pointer;
}

.flashcard-inner {
  position: relative;
  width: 100%;
  height: 100%;
  min-height: 160px;
  transition: transform 0.5s;
  transform-style: preserve-3d;
}

.flashcard.flipped .flashcard-inner { transform: rotateY(180deg); }

.flashcard-face {
  position: absolute;
  inset: 0;
  backface-visibility: hidden;
  display: flex;
  align-items: center;
  justify-content: center;
  text-align: center;
  padding: 1.4rem;
  border-radius: var(--radius);
  border: 1px solid var(--border);
  font-size: 1.05rem;
}

.flashcard-face.front { background: var(--accent-soft); font-weight: 700; }
.flashcard-face.back { background: var(--card); transform: rotateY(180deg); }

.flashcard-controls {
  display: flex;
  align-items: center;
  gap: 0.65rem;
  flex-wrap: wrap;
  justify-content: center;
}

.flashcard-progress { color: var(--ink-soft); font-size: 0.9rem; }

.quiz-progress { color: var(--ink-soft); font-size: 0.9rem; margin-bottom: 0.75rem; }
.quiz-question { font-weight: 700; margin-bottom: 0.9rem; }

.quiz-choices { display: flex; flex-direction: column; gap: 0.55rem; margin-bottom: 1rem; }

.quiz-choice {
  text-align: left;
  padding: 0.65rem 0.9rem;
  border-radius: 10px;
  border: 1px solid var(--border);
  background: var(--card);
}
.quiz-choice.correct { background: #e5f3ea; border-color: var(--good); color: var(--good); }
.quiz-choice.incorrect { background: #fbe8e5; border-color: var(--bad); color: var(--bad); }
.quiz-choice.disabled { cursor: default; }

.quiz-explanation {
  background: var(--accent-soft);
  border-radius: 10px;
  padding: 0.75rem 1rem;
  margin-bottom: 1rem;
  font-size: 0.95rem;
}

.quiz-score { text-align: center; padding: 1.5rem; }
.quiz-score .score-big {
  font-size: 2.2rem;
  font-weight: 800;
  color: var(--accent);
}

.empty-state { color: var(--ink-soft); text-align: center; padding: 2rem; }

.katex { font-size: 1.08em; }