HTML Code

<!DOCTYPE html>

<html lang="en">

<head>

  <meta charset="utf-8">

  <title>jQuery .toggleClass dark mode</title>

  <style>

    body {

      font-family: sans-serif; padding: 24px;

      background: #f8fafc; color: #1e293b;

      transition: background 0.2s, color 0.2s;

    }

    body.dark-theme { background: #0f172a; color: #e2e8f0; }

    .card {

      max-width: 320px; padding: 16px; margin-top: 12px;

      border: 1px solid #cbd5e1; border-radius: 8px;

      background: #fff;

    }

    body.dark-theme .card { background: #1e293b; border-color: #475569; }

    button { padding: 8px 16px; cursor: pointer; border-radius: 6px; }

  </style>

</head>

<body>

  <button type="button" id="themeBtn">Toggle dark mode</button>

  <div class="card">

    <h3>Theme demo</h3>

    <p>Click the button to flip the dark-theme class on body.</p>

  </div>

  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>

  <script>

    $("#themeBtn").on("click", function() {

      $("body").toggleClass("dark-theme");

    });

  </script>

</body>

</html>

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jQuery .toggleClass() — dark mode switch (Try It)