:root {
  --fg: #1d1d1f;
  --muted: #6e6e73;
  --border: #e8e8ed;
  --bg: #ffffff;
  --soft: #f5f5f7;
}
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
body { font-family: 'Inter', sans-serif; font-size: 15px; line-height: 1.65; color: var(--fg); background: var(--bg); }
a { color: inherit; }

.top { display: flex; justify-content: center; padding: 14px 16px; border-bottom: 1px solid var(--border); }
.logo img { height: 30px; width: auto; display: block; }

.wrap { max-width: 720px; margin: 0 auto; padding: 24px 16px 56px; }
.back { display: inline-block; font-size: 13px; color: var(--muted); text-decoration: none; margin-bottom: 20px; }
.back:hover { color: var(--fg); }
h1 { font-size: 28px; line-height: 1.2; font-weight: 700; margin-bottom: 20px; letter-spacing: -0.02em; }
h2 { font-size: 17px; font-weight: 600; margin: 28px 0 8px; }
p { margin-bottom: 12px; }
ul, ol { margin: 0 0 12px 20px; }
li { margin-bottom: 6px; }
.muted { color: var(--muted); font-size: 13px; }

.card { background: var(--soft); border-radius: 14px; padding: 18px 20px; margin: 16px 0; }
.card p:last-child { margin-bottom: 0; }

.btn { display: inline-block; background: #000; color: #fff; text-decoration: none; font-weight: 600; padding: 13px 26px; border-radius: 14px; margin: 6px 0 8px; }
.btn:hover { background: #333; }

details { border-bottom: 1px solid var(--border); padding: 12px 0; }
summary { cursor: pointer; font-weight: 500; list-style: none; display: flex; justify-content: space-between; gap: 12px; }
summary::-webkit-details-marker { display: none; }
summary::after { content: "+"; font-size: 18px; line-height: 1; color: var(--muted); }
details[open] summary::after { content: "−"; }
details p { margin: 8px 0 0; color: #3a3a3c; }

mark.todo { background: #fff3b0; color: #6b4e00; padding: 1px 5px; border-radius: 4px; font-size: 0.92em; }

.foot { border-top: 1px solid var(--border); padding: 24px 16px 32px; text-align: center; font-size: 12px; color: var(--muted); }
.foot nav { display: flex; flex-wrap: wrap; justify-content: center; gap: 6px 16px; max-width: 720px; margin: 0 auto 14px; }
.foot nav a { color: var(--muted); text-decoration: none; }
.foot nav a:hover { color: var(--fg); }

@media (max-width: 480px) {
  h1 { font-size: 24px; }
}