In the html header you would have a link to your stylesheet (with an ID value) like this:
Code:
<link rel="stylesheet" href="style1.css" id="stylesheet">
Also in the header, you would create this javascript code:
Code:
<script type="text/javascript">
<!--
function changeStyles() {
document.getElementById("stylesheet").href = "style2.css";
}
//-->
</script>
Then, on the button you click on the site to change the stylesheet, you would include this code inside the button/image/whatever they're clicking on:
Code:
onClick="changeStyles();"