Examine source code of testing-addon

{
  "name": "testing-addon",
  "manifest_version": 2,
  "version": "1.0",
  "theme": {
    "images": {
      "theme_frame": "gura night theme.png"
    },
    "colors": {
      "frame": "rgba(229, 230, 232, 1)",
      "tab_background_text": "rgba(0, 0, 0, 1)"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{ce123181-a928-48db-a293-fb85c24bfbca}"
    }
  }
}