Examine source code of Container Spaces

{
  "manifest_version": 2,
  "name": "Container Spaces",
  "description": "Allow setting container-specific themes.",
  "version": "0.0.2",
  "browser_specific_settings": {
    "gecko": {
      "id": "container-spaces@different55.imconfusedbythisidform.at"
    }
  },
  "icons": {
    "64": "icons/icon.png"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "permissions": [
    "storage",
    "contextualIdentities",
    "cookies",
    "theme"
  ]
}