
function getDefaultTheme()
{
var linkElements = document.getElementsByTagName("link");
var i;
for (i=0; i < linkElements.length; i++)
{
var linkElement = linkElements[i];
if ((linkElement.getAttribute("rel").indexOf("style") != -1) &&
(linkElement.getAttribute("rel").indexOf("alt") == -1) &&
linkElement.getAttribute("title"))
{
return linkElement.getAttribute("title");
}
}
return null;
}

function setTheme(themeName)
{
var linkElements = document.getElementsByTagName("link");
var i;
for (i=0; i < linkElements.length; i++)
{
var linkElement = linkElements[i];
if ((linkElement.getAttribute("rel").indexOf("style") != -1) &&
linkElement.getAttribute("title"))
{
linkElement.disabled = true;
if (linkElement.getAttribute("title") == themeName)
{
linkElement.disabled = false;
}
}
}
var date = new Date();
date.setTime(date.getTime() + (365*24*60*60*1000));
var expires = "; expires=" + date.toGMTString();
document.cookie = ".Theme" + "=" + themeName + expires +"; path=/";
}

function onThemeButtonClick(themeName)
{
setTheme(themeName);
}

function onWindowLoad()
{
var themeName = null;
var cookies = document.cookie.split(';');
for (var i = 0; i < cookies.length; i++)
{
var cookie = cookies[i];
var cookieParts = cookie.split('=');
if (cookieParts[0].indexOf('.Theme') != -1)
{
themeName = cookieParts[1];
break;
}
}
if (themeName == null)
{
themeName = getDefaultTheme();
}
if (themeName != null)
{
setTheme(themeName);
}
}
window.onload = onWindowLoad;
<link href="/Themes/Core.css" type="text/css" rel="stylesheet" />
<link href="/Themes/Dark/Theme.css" type="text/css" title="Dark" rel="stylesheet" />
<link href="/Themes/Light/Theme.css" type="text/css" title="Light" rel="alternate stylesheet" />