:root {
  color-scheme: light;
  --page-bg: #ffffff;
  --text: #1f2937;
  --muted: #64748b;
  --line: #dbe3f0;
  --panel: #ffffff;
}

* {
  box-sizing: border-box;
}

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

.page {
  width: min(1220px, calc(100% - 24px));
  margin: 0 auto;
  padding: 20px 0 28px;
}

.page-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
  margin-bottom: 12px;
}

h1 {
  margin: 0;
  font-size: clamp(1.375rem, 2.5vw, 1.875rem);
  line-height: 1.15;
  letter-spacing: -0.02em;
  font-weight: 600;
}

.back-link {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  min-height: 36px;
  padding: 0 12px;
  border: 1px solid var(--line);
  background: #ffffff;
  color: var(--text);
  text-decoration: none;
  font-size: 0.95rem;
  font-weight: 600;
  white-space: nowrap;
}

.back-link i {
  width: 14px;
  height: 14px;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  color: var(--muted);
}

.back-link i svg {
  width: 14px;
  height: 14px;
  display: block;
}

.courses-shell {
  min-height: 420px;
  border: 1px solid var(--line);
  background: var(--panel);
  padding: 20px;
}

.courses-stack {
  display: grid;
  gap: 14px;
}

.course-card {
  border: 1px solid var(--line);
  background: #ffffff;
  overflow: hidden;
}

.course-summary {
  list-style: none;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
  padding: 14px 16px;
  cursor: pointer;
}

.course-summary::-webkit-details-marker {
  display: none;
}

.course-summary-main {
  min-width: 0;
  display: grid;
  gap: 2px;
}

.course-title {
  display: block;
  font-size: 1rem;
  font-weight: 800;
  letter-spacing: -0.01em;
}

.course-subtitle {
  display: block;
  font-size: 0.9rem;
  color: var(--muted);
  font-weight: 600;
}

.course-count {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  flex: 0 0 auto;
  padding-left: 12px;
  color: var(--muted);
  font-weight: 700;
}

.course-count i {
  width: 14px;
  height: 14px;
  display: inline-flex;
  align-items: center;
  justify-content: center;
}

.course-count i svg {
  width: 14px;
  height: 14px;
  display: block;
}

.course-body {
  padding: 0 16px 16px;
  border-top: 1px solid var(--line);
}

.video-grid {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: 18px;
  padding-top: 16px;
}

.video-card {
  border: 1px solid var(--line);
  background: #ffffff;
  overflow: hidden;
}

.video-meta {
  display: flex;
  align-items: center;
  justify-content: flex-end;
  padding: 10px 12px;
  border-bottom: 1px solid var(--line);
  background: #ffffff;
}

.video-check {
  display: inline-flex;
  align-items: center;
  gap: 10px;
  font-size: 0.9rem;
  font-weight: 700;
  color: var(--muted);
  user-select: none;
}

.video-check-input {
  width: 18px;
  height: 18px;
  margin: 0;
  cursor: pointer;
  accent-color: #16a34a;
}

.video-frame {
  aspect-ratio: 16 / 9;
  width: 100%;
  border: 0;
}

@media (max-width: 900px) {
  .page {
    width: min(100%, calc(100% - 20px));
    padding-top: 16px;
  }

  .page-header {
    flex-direction: column;
    align-items: flex-start;
  }

  .courses-shell {
    padding: 14px;
  }

  .course-summary {
    align-items: flex-start;
    flex-direction: column;
  }

  .course-count {
    padding-left: 0;
  }

  .video-grid {
    grid-template-columns: 1fr;
    gap: 14px;
  }
}