:root {
  color-scheme: light;
  --primary: #2563eb;
  --text: #0f172a;
  --text-light: #475569;
  --bg: #f8fafc;
  --card: #ffffff;
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  font-family: "Noto Sans SC", system-ui, -apple-system, sans-serif;
  color: var(--text);
  background: var(--bg);
}

.tool-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 1.2rem 5vw;
  background: #ffffff;
  border-bottom: 1px solid rgba(148, 163, 184, 0.3);
  position: sticky;
  top: 0;
  z-index: 10;
}

.brand {
  font-weight: 700;
  text-decoration: none;
  color: var(--text);
}

.tool-nav a {
  margin-left: 1rem;
  text-decoration: none;
  color: var(--text-light);
  font-weight: 500;
}

.container {
  padding: 2.5rem 5vw 4rem;
  max-width: 1100px;
  margin: 0 auto;
}

.intro h1 {
  font-size: clamp(2rem, 3vw, 2.6rem);
  margin-bottom: 0.5rem;
}

.intro p {
  margin: 0;
  color: var(--text-light);
}

.age-filter {
  margin-top: 1.25rem;
  display: flex;
  flex-wrap: wrap;
  gap: 0.75rem;
  align-items: flex-end;
}

.age-filter label {
  display: flex;
  flex-direction: column;
  gap: 0.4rem;
  font-weight: 600;
}

.age-filter select {
  padding: 0.6rem 0.75rem;
  border-radius: 12px;
  border: 1px solid rgba(148, 163, 184, 0.4);
  font-size: 0.95rem;
}

.primary {
  padding: 0.7rem 1.4rem;
  border-radius: 999px;
  border: none;
  background: var(--primary);
  color: #fff;
  font-weight: 600;
  cursor: pointer;
  box-shadow: 0 16px 30px rgba(37, 99, 235, 0.3);
}

.age-sections {
  margin-top: 2.5rem;
  display: grid;
  gap: 1.5rem;
}

.age-card.hidden {
  display: none;
}

.age-card {
  background: var(--card);
  border-radius: 20px;
  padding: 1.5rem;
  border: 1px solid rgba(148, 163, 184, 0.2);
  box-shadow: 0 18px 40px rgba(15, 23, 42, 0.08);
}

.age-card h2 {
  margin-top: 0;
  margin-bottom: 0.5rem;
}

.age-card p {
  margin: 0 0 1rem;
  color: var(--text-light);
}

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

.book-item {
  padding: 1rem;
  border-radius: 16px;
  background: rgba(37, 99, 235, 0.08);
  border: 1px solid rgba(37, 99, 235, 0.12);
}

.book-item h3 {
  margin: 0 0 0.4rem;
  font-size: 1rem;
}

.book-item span {
  display: inline-block;
  margin-top: 0.5rem;
  font-size: 0.8rem;
  color: var(--primary);
  font-weight: 600;
}

@media (max-width: 640px) {
  .tool-header {
    flex-direction: column;
    align-items: flex-start;
    gap: 0.75rem;
  }

  .tool-nav a {
    margin-left: 0;
    margin-right: 1rem;
  }
}