var Win = navigator.appVersion.indexOf('Win',0) != -1;
var Mac = navigator.appVersion.indexOf('Mac',0) != -1;

if(Mac){ document.writeln('<link rel="stylesheet" type="text/css" href="/css/style.css" >'); }
if(Win){ document.writeln('<link rel="stylesheet" type="text/css" href="/css/stylewin.css" >'); }