<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://ваш сайт/'+diz+'.css">');  

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

Выберите дизайн сайта:  
<select name="mdiz">  
<option value="styles1" selected="selected">Темный</option>  
<option value="styles2">Светлый</option>  
</select>  

<input type="button" name="save" onclick="dizsave(); return true;" value="Сохранить" />