var version = navigator.appVersion;

if (version.indexOf("Mac") != -1) {
  document.write('<link rel=stylesheet href=\"css/help.css\" TYPE=\"text/css\">');
}
else if (version.indexOf("Windows") != -1) {
  document.write('<link rel=stylesheet href=\"css/help.css\" TYPE=\"text/css\">');
}
else {
  document.write('<link rel=stylesheet href=\"css/help.css\" TYPE=\"text/css\">');                         
}