:root {
  --paper: #faf9f6;
  --ink: #22262b;
  --ink-soft: #565d66;
  --rule: #ddd9d0;
  --teal: #2a6266;
  --teal-dark: #1b4548;
  --blue: #2b529a;
  --blue-dark: #244480;
  --gold: #a9803f;
  --pdf: #8a3b3b;

  --serif: Palatino, "Palatino Linotype", "Iowan Old Style", Georgia, serif;
  --sans: -apple-system, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  --mono: ui-monospace, SFMono-Regular, Menlo, Consolas, "Liberation Mono", monospace;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  padding: 0 1.25rem 5rem;
  background: var(--paper);
  color: var(--ink);
  font-family: var(--sans);
  font-size: 17px;
  line-height: 1.55;
}

main {
  max-width: 640px;
  margin: 0 auto;
}

header {
  max-width: 640px;
  margin: 0 auto;
  padding-top: 2rem;
}

a {
  color: var(--teal-dark);
  text-decoration-color: var(--rule);
  text-underline-offset: 0.15em;
}

a:hover, a:focus-visible {
  color: var(--teal);
  text-decoration-color: currentColor;
}

:focus-visible {
  outline: 2px solid var(--teal);
  outline-offset: 2px;
}

h1, h2, h3 {
  font-family: var(--serif);
  font-weight: 600;
  color: var(--ink);
  line-height: 1.2;
}

h1 {
  font-size: 1.9rem;
  margin: 0.4rem 0 1.6rem;
  border-bottom: 2px solid var(--teal);
  padding-bottom: 0.6rem;
}

h2 {
  font-size: 1.3rem;
  margin: 2.6rem 0 0.9rem;
  padding-bottom: 0.35rem;
  border-bottom: 1px solid var(--rule);
}

h3 {
  font-size: 1.02rem;
  font-weight: 600;
  margin: 1.6rem 0 0.6rem;
  color: var(--teal-dark);
}

section:first-of-type h2 { margin-top: 0; }

p { margin: 0.7rem 0; }

/* --- notice banner --- */

.notice {
  font-family: var(--sans);
  font-size: 0.92rem;
  color: var(--ink-soft);
  background: #fff;
  border-left: 3px solid var(--gold);
  padding: 0.7rem 1rem;
  margin: 0 0 1.4rem;
}

.notice a { color: var(--teal-dark); }

/* --- textbook covers ---
   anchors are sized to the image only, so the clickable area
   never spills into the surrounding whitespace. */

ul.textbooks {
  list-style: none;
  display: flex;
  flex-wrap: wrap;
  gap: 2rem;
  margin: 1rem 0;
  padding: 0;
}

ul.textbooks li {
  flex: 0 1 155px;
  text-align: center;
}

ul.textbooks a {
  display: inline-block;
  text-decoration: none;
}

ul.textbooks img {
  display: block;
  width: 155px;
  height: auto;
  border: 1px solid var(--rule);
  box-shadow: 3px 3px 0 var(--rule);
}

ul.textbooks a:hover img,
ul.textbooks a:focus-visible img {
  border-color: var(--teal);
}

ul.textbooks p {
  font-size: 0.9rem;
  margin-top: 0.6rem;
  color: var(--ink-soft);
}

/* --- video list --- */

p.section-label {
  font-weight: 600;
  font-size: 0.85rem;
  color: var(--ink-soft);
  margin: 1.5rem 0 0.3rem;
}

ul.videos {
  list-style: none;
  margin: 0 0 0.8rem;
  padding: 0;
}

ul.videos li {
  padding: 0.45rem 0 0.45rem 0.9rem;
  border-bottom: 1px solid var(--rule);
}

.tag {
  display: inline-block;
  font-family: var(--mono);
  font-size: 0.7rem;
  letter-spacing: 0.02em;
  padding: 0.1em 0.45em;
  margin-left: 0.5em;
  border-radius: 3px;
  color: var(--ink-soft);
  border: 1px solid var(--rule);
}

.tag-warn {
  color: var(--gold);
  border-color: var(--gold);
}

/* --- lecture notes --- */

p.note {
  font-size: 0.9rem;
  color: var(--ink-soft);
  border-left: 3px solid var(--rule);
  padding-left: 0.9rem;
}

.lecture-group {
  padding-left: 1rem;
  xborder-left: 2px solid var(--rule);
}

ul.files {
  list-style: none;
  margin: 0;
  padding: 0;
}

ul.files li {
  margin: 0.4rem 0;
}

ul.files a {
  text-decoration: none;
  display: inline-flex;
  align-items: baseline;
  gap: 0.55em;
}

ul.files a:hover,
ul.files a:focus-visible {
  text-decoration: underline;
}

.filetype {
  font-family: var(--mono);
  font-size: 0.68rem;
  letter-spacing: 0.02em;
  padding: 0.12em 0.4em;
  border-radius: 3px;
  color: #fff;
  flex: none;
  min-width: 3.5em;
  text-align: center;
}

.filetype.pdf { background: var(--pdf); }
.filetype.lean { background: var(--teal); }
.filetype.video { background: var(--blue); font-size: 1rem; }

@media (max-width: 480px) {
  body { font-size: 16px; padding: 0 1rem 4rem; }
  h1 { font-size: 1.6rem; }
  ul.textbooks { gap: 1.4rem; justify-content: center; }
}