HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>modern data-theme</title>
  <style>
    body{font-family:system-ui,sans-serif;padding:1.5rem;}
    :root[data-theme="dark"] body{background:#111;color:#eee;}
    :root[data-theme="light"] body{background:#fff;color:#111;}
    button{margin-right:0.5rem;}
    pre{border:1px solid #ccc;padding:0.75rem;}
  </style>
</head>
<body>
  <button type="button" data-theme="light">Light</button>
  <button type="button" data-theme="dark">Dark</button>
  <pre id="out">Pick a theme</pre>
  <script>
    function setTheme(name) {
      document.documentElement.dataset.theme = name;
      document.getElementById("out").textContent = "theme: " + name;
    }
    document.querySelectorAll("button").forEach(function (btn) {
      btn.addEventListener("click", function () {
        setTheme(btn.getAttribute("data-theme"));
      });
    });
  </script>
</body>
</html>

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