userAgt = navigator.userAgent;

loadcss = " <link rel='stylesheet' type='text/css' href=' ";

if(userAgt.indexOf('Mac') != -1){
loadcss+=" http://www.laloopa.com/myBlog/wp-content/themes/modicus/style.css' /> "; 
} else  {
loadcss+=" http://www.laloopa.com/myBlog/wp-content/themes/modicus/style_win.css' /> ";
}
document.write(loadcss);