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>");