Examine source code of www.facebook.com

{
  "theme": {
    "images": {
      "theme_frame": "header.png"
    },
    "colors": {
      "frame": "#e6e6e6",
      "tab_background_text": "#e6e6e6"
    }
  },
  "version": "2.0",
  "name": "www.facebook.com",
  "manifest_version": 2,
  "description": "The skin is made under the official version of Firefox.For monitors with a resolution of 1600X900 pixel,with a menu bar and bookmarks toolbar."
}