Here's an example of the way I do stuff like this.
Demo with the background-color, pure JS, no server stuff.
(Drop it on the bottom of a skin and it should work, I tested with the 4.9 default skin)
<a href=" loadCssFile ('<%=SkinPath%>Blue.css');">Blue</a>
<a href=" loadCssFile ('<%=SkinPath%>Red.css');">Red</a>
<a href=" loadCssFile ('<%=SkinPath%>Green.css');">Green</a>
<script type="text/javascript">
var sCookie = "UserStyleSheet" //Name of user style sheet Cookie
var sCssId = "CustomStyle"
var oLastCSS = readCookie (sCookie); // Get the last stylesheet from the cookie
if (undefined != oLastCSS){loadCssFile (oLastCSS);} // Load the last stylesheet saved in the Cookie
function loadCssFile(filename){
//Load a stylesheet and saves in a Cookie
createCookie (sCookie, filename, 365);
var oStyleSheet = document.getElementById(sCssId)
if (undefined != oStyleSheet) { // If the style link was already added, change the Href
oStyleSheet.setAttribute("href", filename)}
else{ // Add a Css link
var fileref=document.createElement("link");
fileref.setAttribute("rel", "stylesheet");
fileref.setAttribute("type", "text/css");
fileref.setAttribute("href", filename);
fileref.setAttribute("id", sCssId);
document.getElementsByTagName("head")[0].appendChild(fileref);
}
}
function createCookie(name,value,days) {
if (days) {
var date = new Date();
date.setTime(date.getTime()+(days*24*60*60*1000));
var expires = "; expires="+date.toGMTString();
}
else var expires = "";
document.cookie = name+"="+value+expires+"; path=/";
}
function readCookie(name) {
var nameEQ = name + "=";
var ca = document.cookie.split(';');
for(var i=0;i < ca.length;i++) {
var c = ca[i];
while (c.charAt(0)==' ') c = c.substring(1,c.length);
if (c.indexOf(nameEQ) == 0) return c.substring(nameEQ.length,c.length);
}
return null;
}
function eraseCookie(name) {
createCookie(name,"",-1);
}
</script>
The other thing you need is 3 CSS files, Green, Red and Blue.css, containing this:
body{background:#D00 none !important;} (with the right color)
in the root of the skin folder