s_sw=screen.width;
s_sh=screen.height;
if (s_sw <= 1024)  {
  document.write('<link rel="stylesheet" type="text/css" href="'+theStylePath+'/1024.css">');
}
else if (s_sw >= 1280)  {
  document.write('<link rel="stylesheet" type="text/css" href="'+theStylePath+'/1280.css">');
}
else {
  document.write('<link rel="stylesheet" type="text/css" href="'+theStylePath+'/1024.css">');
}


Read more: http://www.btiteam.org/smf/index.php?topic=12067.0#ixzz0kmUDEyXS