Examine source code of Thunderbird 16

{
  "theme": {
    "images": {
      "theme_frame": "Thunderbirdheader.png"
    },
    "colors": {
      "frame": "#e9f2fe",
      "tab_background_text": "#000000"
    }
  },
  "version": "2.0",
  "name": "Thunderbird 16",
  "manifest_version": 2,
  "description": "Matches the design of Thunderbird 16 (and possibly earlier versions)"
}