index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <title>Dark Mode Toggle</title>
    <meta charset="utf-8">
    <meta name="description" content="Dark Mode Toggle Example">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
 	<!-- Loading manifest -->
 		<link rel="manifest" href="../manifest.json">
 <!-- Loading third party fonts -->
	<link rel="stylesheet" href="css/css.css">

</head>


<body>

    <script>
        var app = document.getElementsByTagName("BODY")[0];
        if (localStorage.lightMode == "dark") {
            app.setAttribute("light-mode", "dark");
        }
    </script>

    <script>
		
        function toggle_light_mode() {
            var app = document.getElementsByTagName("BODY")[0];
            if (localStorage.lightMode == "dark") {
                localStorage.lightMode = "light";
                app.setAttribute("light-mode", "light");
            } else {
                localStorage.lightMode = "dark";
                app.setAttribute("light-mode", "dark");
            }       
        }

		window.addEventListener("storage", function () {
			if (localStorage.lightMode == "dark") {
				app.setAttribute("light-mode", "dark");
			} else {
				app.setAttribute("light-mode", "light");
			}
		}, false);
		
    </script>

</body>
</html>