How to not load the theme css when creating a TinyMCE plugin

ed.windowManager.open({
  ...
  file : url + '/dialog.htm',
  width : 640 + parseInt(ed.getLang('attachment_fu.delta_width', 0)),
  height : 485 + parseInt(ed.getLang('attachment_fu.delta_height', 0)),
  popup_css : false,
  ...
})