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

if (Mac) {
 document.write('<link rel="stylesheet" href="styles/mac.css" type="text/css">');
} else if (Win) {
 document.write('<link rel="stylesheet" href="styles/win.css" type="text/css">');
}