HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>theme class toggle</title>
  <style>
    :root { color-scheme: light; background: #fff; color: #111; }
    :root.dark { color-scheme: dark; background: #111; color: #f5f5f5; }
  </style>
</head>
<body>
  <p><button id="go" type="button">Toggle theme</button></p>
  <pre id="out">theme: light</pre>
  <script>
    var out = document.getElementById("out");
    document.getElementById("go").addEventListener("click", function () {
      document.documentElement.classList.toggle("dark");
      var isDark = document.documentElement.classList.contains("dark");
      out.textContent = "theme: " + (isDark ? "dark" : "light");
    });
  </script>
</body>
</html>

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Modern theme class toggle (Try It)