body {
  font-family: Arial, sans-serif;
  margin: 0; padding: 0;
  background: #fff; color: #111;
  transition: background 0.3s, color 0.3s;
}
header {
  display: flex; justify-content: space-between; align-items: center;
  padding: 1rem; background: #f5f5f5;
}
header nav a {
  margin: 0 0.5rem; text-decoration: none; color: inherit;
}
.dark {
  background: #111; color: #eee;
}
.dark header { background: #222; }
button#darkToggle { cursor: pointer; padding: 0.5rem; }