Examine source code of PortableThunderbird

{
  "theme": {
    "images": {
      "theme_frame": "header.png"
    },
    "colors": {
      "frame": "#5a89cc",
      "tab_background_text": "#f5f7fa"
    }
  },
  "version": "2.0",
  "name": "PortableThunderbird",
  "manifest_version": 2
}