:root {
  --bg: #fffff8;
  --fg: #111;
  --muted: #666;
  --accent: #b45a3c;
  --accent-light: #f3e7e3;
  --border: #ddd;
  --code-bg: #f5f5f0;
  --font-serif: "ETBook", "Palatino Linotype", "Book Antiqua", "Noto Serif CJK SC", "Source Han Serif SC", Georgia, serif;
  --font-sans: "Gill Sans", "Noto Sans CJK SC", "Source Han Sans SC", "Helvetica Neue", Arial, sans-serif;
  --font-mono: "SF Mono", "Fira Code", "Noto Sans Mono CJK SC", Consolas, monospace;
}

* { box-sizing: border-box; }

html {
  font-size: 16px;
  -webkit-font-smoothing: antialiased;
}

body {
  margin: 0;
  padding: 0;
  background: var(--bg);
  color: var(--fg);
  font-family: var(--font-serif);
  line-height: 1.65;
}

.container {
  max-width: 720px;
  margin: 0 auto;
  padding: 2.5rem 1.5rem 4rem;
}

header {
  margin-bottom: 2.5rem;
  border-bottom: 1px solid var(--border);
  padding-bottom: 1.25rem;
}

header h1 {
  font-family: var(--font-sans);
  font-size: 2rem;
  font-weight: 700;
  margin: 0 0 0.5rem;
  letter-spacing: -0.02em;
  line-height: 1.2;
}

header .subtitle {
  color: var(--muted);
  font-size: 1.05rem;
  font-family: var(--font-serif);
  font-style: italic;
}

h2, h3, h4 {
  font-family: var(--font-sans);
  font-weight: 600;
  margin-top: 2.2rem;
  margin-bottom: 0.8rem;
  line-height: 1.3;
}

h2 { font-size: 1.5rem; color: var(--accent); }
h3 { font-size: 1.2rem; }
h4 { font-size: 1.05rem; }

p, li {
  font-size: 1.05rem;
  margin: 0 0 1rem;
}

a {
  color: var(--accent);
  text-decoration: none;
  border-bottom: 1px solid transparent;
}

a:hover { border-bottom-color: var(--accent); }

code, pre {
  font-family: var(--font-mono);
  font-size: 0.9em;
}

code {
  background: var(--code-bg);
  padding: 0.15rem 0.35rem;
  border-radius: 3px;
}

pre {
  background: var(--code-bg);
  padding: 1rem 1.25rem;
  border-radius: 6px;
  overflow-x: auto;
  line-height: 1.5;
  margin: 1.25rem 0;
}

pre code {
  background: transparent;
  padding: 0;
}

.highlight-box {
  background: var(--accent-light);
  border-left: 4px solid var(--accent);
  padding: 1rem 1.25rem;
  margin: 1.5rem 0;
  border-radius: 0 6px 6px 0;
}

.highlight-box p:last-child { margin-bottom: 0; }

.formula {
  background: #fff;
  border: 1px solid var(--border);
  padding: 1rem 1.25rem;
  margin: 1.25rem 0;
  border-radius: 6px;
  text-align: center;
  font-size: 1.1rem;
  overflow-x: auto;
}

table {
  width: 100%;
  border-collapse: collapse;
  margin: 1.5rem 0;
  font-size: 0.95rem;
}

th, td {
  border-bottom: 1px solid var(--border);
  padding: 0.6rem 0.75rem;
  text-align: left;
  vertical-align: top;
}

th {
  font-family: var(--font-sans);
  font-weight: 600;
  background: #fafaf8;
}

tr:last-child td { border-bottom: none; }

.sidenote, .margin-note {
  float: right;
  clear: right;
  width: 220px;
  margin-right: -260px;
  font-size: 0.85rem;
  color: var(--muted);
  line-height: 1.5;
}

@media (max-width: 1100px) {
  .sidenote, .margin-note {
    float: none;
    width: auto;
    margin: 1rem 0;
    display: block;
    color: var(--muted);
  }
}

.quiz {
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 1.25rem;
  margin: 2rem 0;
  background: #fff;
}

.quiz h3 { margin-top: 0; }

.quiz-options button {
  display: block;
  width: 100%;
  text-align: left;
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 0.75rem 1rem;
  margin: 0.5rem 0;
  font-family: var(--font-serif);
  font-size: 1rem;
  cursor: pointer;
  transition: background 0.15s;
}

.quiz-options button:hover { background: var(--accent-light); }

.quiz-options button.correct {
  background: #e6f4ea;
  border-color: #34a853;
}

.quiz-options button.wrong {
  background: #fce8e6;
  border-color: #ea4335;
}

.quiz-feedback {
  margin-top: 1rem;
  padding: 0.75rem 1rem;
  border-radius: 6px;
  display: none;
}

.quiz-feedback.show { display: block; }

footer {
  margin-top: 4rem;
  padding-top: 1.5rem;
  border-top: 1px solid var(--border);
  color: var(--muted);
  font-size: 0.9rem;
}

footer a { color: var(--muted); }

.lesson-nav {
  display: flex;
  justify-content: space-between;
  flex-wrap: wrap;
  gap: 0.5rem;
  margin-top: 3rem;
  padding-top: 1rem;
  border-top: 1px solid var(--border);
  font-family: var(--font-sans);
  font-size: 0.9rem;
}

@media print {
  body { background: #fff; }
  .quiz-options button { border: 1px solid #999; }
}