Examine source code of xkcdbis

{
  "theme": {
    "images": {
      "theme_frame": "xkcdtop2.png"
    },
    "colors": {
      "frame": "#cccccc",
      "tab_background_text": "#000000"
    }
  },
  "version": "2.0",
  "name": "xkcdbis",
  "manifest_version": 2,
  "description": "Initially created by 'chintogtokh' (<a href=\"https://outgoing.prod.mozaws.net/v1/f833d2deda0ed2a0d71f4f9cfc14954676050b9bdbbf505babf54c87712ea46f/http%3A//www.getpersonas.com/en-US/gallery/Designer/chintogtokh\" rel=\"nofollow\">http://www.getpersonas.com/en-US/gallery/Designer/chintogtokh</a>). I just inverted the colours to make it brighter."
}