Examine source code of Birds are Angry!

{
  "theme": {
    "images": {
      "theme_frame": "header.png"
    },
    "colors": {
      "frame": "#4a432b",
      "tab_background_text": "#ffffff"
    }
  },
  "version": "2.0",
  "name": "Birds are Angry!",
  "manifest_version": 2,
  "description": "Angry Birds on your Seamonkey / Firefox browser!\n\nVisit my profile and request a theme as you like ;)\n<a rel=\"nofollow\" href=\"https://outgoing.prod.mozaws.net/v1/cf8232736de19c7bad43da39935d5e7b0d096912e288c4288401ea59f2b84ac3/http%3A//www.facebook.com/pychome\">www.facebook.com/pychome</a>"
}