:root {
  --bg: #f4f8fb;
  --surface: #ffffff;
  --text: #2d3f54;
  --muted: #52647a;
  --border: rgba(82, 100, 122, 0.24);
  --blue: #004c94;
  --blue-dark: #004c94;
  --blue-bright: #006ecf;
  --sky: #3c93fa;
  --green: #73cc80;
  --mint: #1dbd8e;
  --cyan: #00d4c6;
  --teal: #0fa69d;
  --deep-teal: #028183;
  --shadow: 0 14px 35px rgba(45, 63, 84, 0.10);
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  min-height: 100vh;
  font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  color: var(--text);
  background: var(--bg);
}

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

.site-header {
  max-width: 1180px;
  margin: 0 auto;
  padding: 28px 20px 14px;
}

.topnav {
  display: flex;
  justify-content: center;
  gap: 10px;
  flex-wrap: wrap;
  margin-bottom: 30px;
}

.topnav a {
  border: 1px solid var(--border);
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.88);
  color: var(--muted);
  font-weight: 800;
  padding: 8px 13px;
}

.topnav a:hover,
.topnav a.active {
  color: #ffffff;
  border-color: var(--blue);
  background: var(--blue);
}

.hero {
  text-align: center;
  padding: 24px 24px 28px;
  border-radius: 8px;
  background: var(--blue);
  color: #ffffff;
  box-shadow: var(--shadow);
}

h1 {
  margin: 0 0 8px;
  font-size: clamp(1.7rem, 3.4vw, 2.9rem);
  font-weight: 500;
  line-height: 1.12;
  letter-spacing: 0;
}

.author,
.subtitle {
  margin: 0 auto;
  max-width: 820px;
  color: var(--muted);
  line-height: 1.55;
}

.hero .author,
.hero .subtitle {
  color: rgba(255, 255, 255, 0.88);
}

.author {
  font-size: clamp(1.05rem, 2vw, 1.35rem);
  font-weight: 400;
}

main {
  max-width: 1180px;
  margin: 0 auto;
  padding: 18px 20px 38px;
}

.grid {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: 16px;
}

.chapters {
  display: grid;
  gap: 18px;
}

.chapter-block {
  background: rgba(255, 255, 255, 0.94);
  border: 1px solid var(--border);
  border-radius: 8px;
  box-shadow: var(--shadow);
  padding: 18px;
}

.chapter-block h2 {
  margin: 0 0 12px;
  font-size: 1.2rem;
}

.exercise-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(230px, 1fr));
  gap: 12px;
}

.card {
  min-height: 172px;
  border: 1px solid var(--border);
  border-radius: 8px;
  background: rgba(255, 255, 255, 0.96);
  box-shadow: var(--shadow);
  padding: 18px;
  display: flex;
  flex-direction: column;
  justify-content: space-between;
  gap: 16px;
  transition: transform 0.15s ease, border-color 0.15s ease, box-shadow 0.15s ease;
}

.card:hover {
  transform: translateY(-2px);
  border-color: var(--sky);
  box-shadow: 0 18px 42px rgba(45, 63, 84, 0.14);
}

.card.python {
  border-top: 4px solid var(--green);
}

.card.cours {
  border-top: 4px solid var(--blue);
}

.card.revisions {
  border-top: 4px solid var(--cyan);
}

.card.exercise {
  min-height: 138px;
  border-top: 4px solid var(--teal);
}

.symbol {
  width: 44px;
  height: 44px;
  border-radius: 8px;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  font-weight: 950;
  color: #ffffff;
  background: var(--blue);
  font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", monospace;
}

.python .symbol {
  background: var(--green);
}

.revisions .symbol {
  background: var(--cyan);
  color: #2d3f54;
}

.exercise .symbol {
  background: var(--teal);
}

.card h2,
.card h3 {
  margin: 0;
  font-size: 1.18rem;
  line-height: 1.25;
}

.card p {
  margin: 6px 0 0;
  color: var(--muted);
  line-height: 1.45;
}

.link-label {
  color: var(--blue);
  font-weight: 900;
}

.notice {
  max-width: 760px;
  margin: 30px auto 0;
  border: 1px solid var(--border);
  border-radius: 8px;
  background: rgba(255, 255, 255, 0.96);
  box-shadow: var(--shadow);
  padding: 28px;
  text-align: center;
}

.notice h2 {
  margin: 0 0 8px;
  font-size: clamp(1.5rem, 3vw, 2.3rem);
}

.notice p {
  margin: 0;
  color: var(--muted);
  font-size: 1.08rem;
  line-height: 1.55;
}

@media (max-width: 820px) {
  .grid {
    grid-template-columns: 1fr;
  }
}