Examine source code of Thunderbird dark

{
  "theme": {
    "images": {
      "theme_frame": "thunderbirdHI.jpg"
    },
    "colors": {
      "frame": "#e0e0e0",
      "tab_background_text": "#ffffff"
    }
  },
  "version": "2.0",
  "name": "Thunderbird dark",
  "manifest_version": 2
}