<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.ru/'+diz+'.css">');   

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

<select name="mdiz">   
<option value="styles1" selected="selected">Название первого дизайна</option>   
<option value="styles2">Название второго дизайна</option>   
</select>   

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