:root {
  color-scheme: light;
  --ink: #172026;
  --muted: #5d6b75;
  --line: #d8e0e7;
  --paper: #ffffff;
  --soft: #f3f7f8;
  --blue: #2357a3;
  --teal: #0f766e;
  --amber: #9a5b00;
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  color: var(--ink);
  background: var(--soft);
  font: 16px/1.55 -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
}

a {
  color: var(--blue);
}

nav {
  display: flex;
  gap: 1rem;
  flex-wrap: wrap;
  margin-bottom: 4rem;
}

nav a {
  color: #e7f3ff;
  text-decoration: none;
  font-weight: 650;
}

.hero {
  min-height: 72vh;
  padding: 2rem clamp(1rem, 4vw, 4rem);
  color: #fff;
  background:
    linear-gradient(rgba(9, 28, 43, 0.72), rgba(9, 28, 43, 0.74)),
    url("https://images.unsplash.com/photo-1516280440614-37939bbacd81?auto=format&fit=crop&w=1800&q=80");
  background-size: cover;
  background-position: center;
  display: flex;
  flex-direction: column;
  justify-content: flex-start;
}

.lesson-hero {
  min-height: 54vh;
  padding: 2rem clamp(1rem, 4vw, 4rem);
  color: #fff;
  background:
    linear-gradient(rgba(12, 36, 48, 0.78), rgba(12, 36, 48, 0.76)),
    url("https://images.unsplash.com/photo-1500530855697-b586d89ba3ee?auto=format&fit=crop&w=1800&q=80");
  background-size: cover;
  background-position: center;
}

.hero h1 {
  max-width: 980px;
  margin: 0;
  font-size: clamp(2.4rem, 7vw, 6.4rem);
  line-height: 0.98;
  letter-spacing: 0;
}

.lede {
  max-width: 760px;
  font-size: clamp(1.1rem, 2vw, 1.45rem);
}

.eyebrow {
  margin: 0 0 0.6rem;
  color: var(--teal);
  font-weight: 750;
  text-transform: uppercase;
  letter-spacing: 0;
}

.hero .eyebrow {
  color: #9ee7dc;
}

.band {
  padding: 4rem clamp(1rem, 4vw, 4rem);
  background: var(--paper);
  border-bottom: 1px solid var(--line);
}

.band:nth-of-type(even) {
  background: #f9fbfc;
}

.section-head {
  max-width: 760px;
  margin-bottom: 2rem;
}

h2 {
  margin: 0;
  font-size: clamp(1.7rem, 3vw, 3rem);
  line-height: 1.05;
  letter-spacing: 0;
}

h3 {
  margin: 0 0 0.6rem;
}

.grid {
  display: grid;
  gap: 1rem;
}

.grid.two {
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
}

.grid.three {
  grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
}

.card {
  border: 1px solid var(--line);
  border-radius: 8px;
  padding: 1.25rem;
  background: #fff;
}

.prompt {
  max-width: 980px;
  margin: 1.25rem 0;
  border-left: 4px solid var(--teal);
  padding: 1rem 1.25rem;
  background: #fff;
}

.problem-list,
.problem-group {
  display: grid;
  gap: 1rem;
}

.problem-group {
  margin: 2rem 0;
}

.problem-card {
  max-width: 980px;
  border: 1px solid var(--line);
  border-radius: 8px;
  padding: 1rem;
  background: #fff;
}

.problem-card h4 {
  margin: 0 0 0.5rem;
  font-size: 1.05rem;
}

.steps {
  max-width: 880px;
  padding-left: 1.4rem;
}

.steps li {
  margin: 0.75rem 0;
}

.timeline {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(320px, 1fr));
  gap: 1rem;
  align-items: stretch;
}

.timeline article {
  display: flex;
  flex-direction: column;
  gap: 0.65rem;
  min-height: 15rem;
  padding: 1.15rem;
  border: 1px solid var(--line);
  border-top: 4px solid var(--teal);
  border-radius: 8px;
  background: #fff;
  box-shadow: 0 10px 24px rgba(23, 32, 38, 0.06);
}

.timeline span {
  width: fit-content;
  min-width: 3.25rem;
  height: 2.25rem;
  padding: 0 0.75rem;
  border-radius: 8px;
  display: grid;
  place-items: center;
  background: #e6f4f1;
  color: var(--teal);
  font-weight: 800;
  font-variant-numeric: tabular-nums;
}

.timeline p {
  margin: 0;
}

.timeline .lab {
  margin-top: auto;
  padding-top: 0.75rem;
  border-top: 1px solid var(--line);
}

.lab {
  color: var(--amber);
  font-weight: 650;
}

.lesson-link {
  display: inline-block;
  margin-top: 0.25rem;
  font-weight: 700;
}

pre {
  overflow-x: auto;
  padding: 1rem;
  border-radius: 8px;
  background: #111827;
  color: #e5e7eb;
}

code {
  font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace;
  font-size: 0.92rem;
}

.qa {
  margin: 1rem 0;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: #fff;
  padding: 0.85rem 1rem;
}

.qa summary {
  cursor: pointer;
  font-weight: 750;
}

.qa p {
  margin-bottom: 0;
}

.checklist {
  max-width: 900px;
  padding-left: 1.2rem;
}

.checklist li {
  margin: 0.55rem 0;
}

.resources {
  max-width: 980px;
  padding-left: 1.2rem;
}

.resources li {
  margin: 0.7rem 0;
}

footer {
  padding: 2rem clamp(1rem, 4vw, 4rem);
  color: var(--muted);
}

@media (max-width: 680px) {
  .hero {
    min-height: 78vh;
  }

  .timeline {
    grid-template-columns: 1fr;
  }
}