if (navigator.platform.substr(0,3).toLowerCase() == "win") {
if (navigator.appName.substr(0,3).toLowerCase() == "net") {
        st = "pc_net";
    } else {
        st = "pc_ex";
    }
} 
else {
    if (navigator.appName.substr(0,3).toLowerCase() == "net") {
        st = "mac";
    } else {
        st = "mac";
    }
}
document.write("<LINK REL=StyleSheet HREF=style_" + st + ".css TYPE=text/css>");