S php niama da stane...opitai tova:
<script language="JavaScript" type="text/javascript">
<!--
if(window.screen.width==800){
documnet.write("mystyle1")
//mystyle moze da bade celia stil ili eventulano da probavsh da napishesh tova s JS
//<link href="mystyle.css" rel="stylesheet" type="text/css">
//viz oba4e svamestomost s NS
}else{
documnet.write("mystyle2")
}
//-->
</script>