var usr_agent = navigator.userAgent.toLowerCase();
var agn_mac = (usr_agent.indexOf('mac') != -1);
var ie = (usr_agent.indexOf('msie') != -1);

if(agn_mac && ie){
    document.write('<link rel="stylesheet" href="css/top-mac.css"  type="text/css" media="screen,print">'); 
}else{
    document.write('<link rel="stylesheet" href="css/top-win.css" type="text/css" media="screen,print">'); 
}