Dark/Light Theme Toggle

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Dark/Light Theme Toggle</title>
<style>
  body {
    /* Initial light theme colors  */
    --bg: #f8fafc;
    --text: #0f172a;
    --btcolor: #6366f1;

    background: var(--bg);
    color: var(--text);
    min-height: 100vh;
    display: flex;
    align-items: center;
    justify-content: center;
    flex-direction: column;
    gap: 2rem;
    transition: background 0.4s ease, color 0.4s ease;
    font-family: system-ui, -apple-system, sans-serif;
  }
  /* This selector handles the dark theme colors */
  body:has(#theme-toggle:checked) {
    --bg: #0f172a;
    --text: #e2e8f0;
    --btcolor: #f59e0b;
  }
  h1 {
    font-size: 2.5rem;
    letter-spacing: -1px;
    transition: color 0.3s ease;
  }
  .toggle-btn {
    padding: 0.75rem 1.5rem;
    border-radius: 0.5rem;
    background: var(--btcolor);
    color: white;
    font-weight: 600;
    letter-spacing: 0.5px;
    cursor: pointer;
    transition: all 0.3s ease;
    user-select: none;
    box-shadow: 0 6px 20px rgba(0,0,0,0.15);

    &:hover {
      transform: translateY(-2px);
      box-shadow: 0 10px 28px rgba(0,0,0,0.2);
    }
  }
</style>
</head>
<body>
  <input style="display: none;" type="checkbox" id="theme-toggle">
  <label for="theme-toggle" class="toggle-btn">
    Toggle Theme
  </label>
  <h1>Hello World</h1>
</body>
<script>
  // Sync checkbox with system theme on load
  window.matchMedia('(prefers-color-scheme: dark)').matches &&
    (document.getElementById('theme-toggle').checked = true);
</script>
</html>