changebackground.html

<html>
<head>
<title>Change Background Colour Javascript Example</title>
</head>
<body>

<h1>Change the Background Colour<h1>

<script type="text/javascript">
var colour = prompt("What is your favourite colour?");
if (colour != "") {
    document.body.style.backgroundColor = colour;
}
</script>

</body>
</html>