Examine source code of Mozilla Thunderbird

{
  "theme": {
    "images": {
      "theme_frame": "ThunderbirdH.jpg"
    },
    "colors": {
      "frame": "#c9c9c9",
      "tab_background_text": "#000000"
    }
  },
  "version": "2.0",
  "name": "Mozilla Thunderbird",
  "manifest_version": 2
}