:root {
  --bg: #f7f5ef;
  --text: #2c2a27;
  --muted: #77716a;
  --accent: #6f5a42;
  --border: #ded8cf;
  --paper: #fffdf8;
}

* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font-family: "Yu Mincho", "Hiragino Mincho ProN", "Hiragino Mincho Pro", "Noto Serif JP", serif;
}

a {
  color: inherit;
  text-decoration: none;
}

.container {
  width: min(40em, calc(100% - 2rem));
  margin: 0 auto;
}

.cover {
  min-height: 100vh;
  display: grid;
  place-items: center;
  padding: 3rem 0;
}

.cover-inner {
  width: min(40em, 100%);
  text-align: center;
}

.title {
  margin: 0 0 1.5rem;
  /* clampの最小値を小さく調整（例: 1.2rem） */
  font-size: clamp(1.2rem, 5vw, 3.4rem);
  font-weight: 500;
  letter-spacing: .08em;
  /* 改行を防止 */
  white-space: nowrap;
}

.subtitle {
  color: var(--muted);
  line-height: 2;
  margin: 0 auto 2.5rem;
  text-align: left;
}

.start {
  display: inline-block;
  border: 1px solid var(--accent);
  padding: .75rem 1.5rem;
  transition: .2s;
}

.start:hover {
  background: var(--accent);
  color: #fff;
}

.chapter-header {
  padding: 4rem 0 2rem;
  text-align: center;
}

.chapter-number {
  color: var(--muted);
  font-size: .9rem;
  letter-spacing: .15em;
}

.chapter-title {
  margin: .7rem 0 0;
  font-size: clamp(1.7rem, 5vw, 2.4rem);
  font-weight: 500;
}

.part {
  background: var(--paper);
  padding: 2rem clamp(1rem, 3vw, 2rem);
  margin: 0 auto 2rem;
  border-top: 1px solid var(--border);
}

.part h2 {
  font-size: 1.2rem;
  font-weight: 500;
  margin: 0 0 2rem;
}

.part p {
  margin: 0 0 1.5em;
  line-height: 2.15;
  text-align: left;
  overflow-wrap: anywhere;
}

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

.nav {
  display: grid;
  grid-template-columns: 1fr auto 1fr;
  gap: .75rem;
  padding: 1rem 0 4rem;
  align-items: center;
  color: var(--muted);
}

.nav a {
  padding: .65rem .4rem;
  text-align: center;
  border-top: 1px solid var(--border);
  border-bottom: 1px solid var(--border);
}

.nav a:hover { color: var(--text); }

.nav .next { text-align: center; }

.footer {
  text-align: center;
  color: var(--muted);
  font-size: .8rem;
  padding: 0 0 2rem;
}

@media (max-width: 600px) {
  .container { width: min(40em, calc(100% - 1.5rem)); }
  .cover { min-height: 92vh; }
  .part { padding: 1.5rem .9rem; }
  .part p { line-height: 2; }
  .nav { font-size: .9rem; }
}