Examine source code of CUSTOM NIGHTLY

{
  "name": "CUSTOM NIGHTLY",
  "manifest_version": 2,
  "version": "1.0",
  "theme": {
    "images": {
      "theme_frame": "FIREFOXTEST.jpg"
    },
    "colors": {
      "frame": "rgba(0, 77, 255, 1)",
      "tab_background_text": "rgba(242, 0, 255, 1)",
      "toolbar": "rgba(0, 0, 0, 1)",
      "bookmark_text": "rgba(242, 0, 255, 1)",
      "toolbar_field": "rgba(255, 255, 255, 1)",
      "toolbar_field_text": "rgba(0, 0, 0, 1)",
      "tab_line": "rgba(0, 255, 242, 1)"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{3f5fa612-0f57-4a09-8b21-755441e04feb}"
    }
  }
}