Examine source code of ThunderBird (Mozilla)

{
  "theme": {
    "images": {
      "theme_frame": "header.png"
    },
    "colors": {
      "frame": "#121111",
      "tab_background_text": "#ffffff"
    }
  },
  "version": "2.0",
  "name": "ThunderBird (Mozilla)",
  "manifest_version": 2,
  "description": "This skin is for Mozilla Thunderbird."
}