В head

[/code]<script>
function getCookie(c_name)
{
if (document.cookie.length>0)
{
c_start=document.cookie.indexOf(c_name + "=");
if (c_start!=-1)
{
c_start=c_start + c_name.length+1;
c_end=document.cookie.indexOf(";",c_start);
if (c_end==-1) c_end=document.cookie.length;
return unescape(document.cookie.substring(c_start,c_end));
}
}
return "";
}

function setCookie(c_name,value,expiredays)
{
var exdate=new Date();
exdate.setDate(exdate.getDate()+expiredays);
document.cookie=c_name+ "=" +escape(value)+
((expiredays==null) ? "" : ";expires="+exdate.toGMTString());
}

var diz = getCookie('mydiz') ? getCookie('mydiz') : 'styles1';
document.write('<link type="text/css" rel="StyleSheet" href="http://proucoz.net/'+diz+'.css">');

function dizsave() {
setCookie('mydiz',document.getElementsByName('mdiz')[0].value, 365);
location.reload();
}
</script>

Куда вам надо:

<select name="mdiz">
<option value="styles1" selected="selected">Темный</option>
<option value="styles2">Светлый</option>
</select>[code]

Теперь заливам в корень свои стили ксс подписуя их styles1.css и styles2.css. готово