popup.html

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <link rel="stylesheet" type="text/css" href="popup.css">
  <title>Dark Whatsapp</title>
</head>

<body>

  <div class="container" id="">

    <div id="header-btn">
      <button class="btn" id="button">Light</button>
    </div>
  </div>

  <script src="popup.js">

    // let element = document.getElementById("button");
    // element.addEventListener("click", () => {
    //   if (element.innerHTML === "Light")
    //     element.innerHTML = "Dark";
    //   else
    //     element.innerHTML = "Light";
    // });


  </script>
</body>

</html>