HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Theme toggle cookie</title>
</head>
<body>
  <h1>Theme preference</h1>
  <button type="button" id="light">Light</button>
  <button type="button" id="dark">Dark</button>
  <pre id="out"></pre>
  <script>
    var thirtyDays = 30 * 24 * 60 * 60;

    function setTheme(mode) {
      document.cookie = "theme=" + mode + "; max-age=" + thirtyDays + "; path=/";
      document.getElementById("out").textContent =
        "Theme updated to: " + mode + "\n\n" + document.cookie;
    }

    document.cookie = "theme=light; max-age=" + thirtyDays + "; path=/";
    document.getElementById("light").addEventListener("click", function () { setTheme("light"); });
    document.getElementById("dark").addEventListener("click", function () { setTheme("dark"); });
    document.getElementById("out").textContent = "Current: light\n\n" + document.cookie;
  </script>
</body>
</html>

Lines: 0 | Characters: 0

Live Preview Auto-refresh on Run

JavaScript Cookies Update — theme toggle (Try It)