co2-4.html

<html lang="en">

<head>
    <title>Background change</title>
</head>
<body>
<center>
    <input id="btn" type="button" value="Change background">
    <script>
        document.getElementById("btn").addEventListener("click", colorchange);
        function colorchange()
        {
            Math.random() * 10000;
            var color = '#' + Math.floor(Math.random() * 10000).toString(16);
            document.body.style.background = color;
        }
    </script>
    </center>
</body>

</html>