:root {
  --primary: #1565c0;
  --primary-dark: #0d47a1;
  --ink: #2d3748;
  --muted: #5f6b7a;
  --surface: #f8f9fb;
  --surface-strong: #eef4fc;
  --border: #e5e7eb;
  --white: #ffffff;
  --shadow: 0 6px 18px rgba(0, 0, 0, 0.08);
}

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

html {
  scroll-behavior: smooth;
}

body {
  font-family: Inter, Arial, sans-serif;
  max-width: 1100px;
  margin: 0 auto;
  padding: 60px 30px 80px;
  background: var(--white);
  color: var(--ink);
  line-height: 1.7;
}

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

img {
  max-width: 100%;
  display: block;
}

header {
  text-align: center;
  margin-bottom: 54px;
}

.site-title {
  font-size: 46px;
  font-weight: 700;
  color: var(--primary);
  margin-bottom: 10px;
}

.tagline {
  font-size: 22px;
  color: var(--muted);
  margin-bottom: 30px;
}

.social {
  display: flex;
  justify-content: center;
  gap: 28px;
}

.social a {
  color: #444;
  font-size: 34px;
  transition: 0.25s ease;
}

.social a:hover {
  color: var(--primary);
  transform: translateY(-3px);
}

.landing-page {
  max-width: 900px;
}

.landing-page main {
  display: block;
}

.landing-page section {
  display: block;
  margin-bottom: 55px;
}

.landing-page .section-title {
  font-size: 24px;
  margin-bottom: 18px;
}

.landing-page .card {
  border-radius: 10px;
  padding: 22px;
}

.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border: 0;
}

nav {
  margin-bottom: 32px;
}

.breadcrumb {
  display: inline-flex;
  flex-wrap: wrap;
  gap: 8px;
  align-items: center;
  color: var(--muted);
  font-size: 15px;
}

.breadcrumb a {
  color: var(--primary);
}

main {
  display: grid;
  gap: 40px;
}

section {
  display: grid;
  gap: 18px;
}

.section-title {
  color: #222;
  font-size: 28px;
  margin-bottom: 6px;
}

.section-copy {
  color: var(--muted);
  max-width: 760px;
}

.grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(260px, 1fr));
  gap: 18px;
}

.card {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 24px;
  transition: transform 0.2s ease, box-shadow 0.2s ease;
}

.card:hover {
  transform: translateY(-4px);
  box-shadow: var(--shadow);
}

.card h3,
.card h4 {
  color: var(--primary);
  margin-bottom: 10px;
  font-size: 20px;
}

.card p {
  color: #555;
  font-size: 15px;
}

.card-link {
  display: block;
  height: 100%;
}

.card-link:hover .card {
  transform: translateY(-4px);
  box-shadow: var(--shadow);
}

.status-pill {
  display: inline-block;
  margin-top: 12px;
  padding: 6px 10px;
  background: var(--surface-strong);
  color: var(--primary-dark);
  border-radius: 999px;
  font-size: 13px;
  font-weight: 600;
  letter-spacing: 0.02em;
}

.button {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 8px;
  margin-top: 14px;
  padding: 10px 14px;
  border-radius: 999px;
  background: var(--primary);
  color: var(--white);
  font-weight: 600;
  border: none;
  cursor: pointer;
}

.button.secondary {
  background: transparent;
  color: var(--primary);
  border: 1px solid var(--primary);
}

.button:hover {
  background: var(--primary-dark);
  color: var(--white);
}

.button:disabled {
  cursor: not-allowed;
  opacity: 0.65;
}

.lesson-page {
  max-width: none;
  padding: 32px 20px 40px;
}

.lesson-frame {
  display: block;
  width: min(1100px, 100%);
  height: min(900px, calc(100vh - 130px));
  min-height: 620px;
  margin: 0 auto;
  border: 1px solid var(--border);
  border-radius: 12px;
}

.lesson-navigation {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  align-items: center;
  width: min(1100px, 100%);
  margin: 24px auto 0;
}

.lesson-navigation .button:first-child {
  justify-self: start;
}

.lesson-navigation .button:nth-child(2) {
  justify-self: center;
}

.lesson-navigation .button:last-child {
  justify-self: end;
}

.button.secondary:hover {
  background: var(--surface-strong);
}

.list {
  display: grid;
  gap: 10px;
  padding-left: 18px;
  color: var(--muted);
}

footer {
  margin-top: 60px;
  padding-top: 24px;
  border-top: 1px solid #ddd;
  text-align: center;
  color: #777;
}

@media (max-width: 700px) {
  body {
    padding: 40px 20px 60px;
  }

  .site-title {
    font-size: 36px;
  }

  .tagline {
    font-size: 18px;
  }

  .social {
    gap: 18px;
  }
}