Examine source code of Customizer

{
  "manifest_version": 3,
  "name": "Customizer",
  "short_name": "Customizer",
  "description": "Add custom CSS to one or many sites",
  "author": "Robustus",
  "icons": {
    "16": "/assets/icon16.png",
    "32": "/assets/icon32.png",
    "48": "/assets/icon48.png",
    "128": "/assets/icon128.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "customizer@robustus.gitlab.io",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "version": "4.1",
  "action": {
    "default_icon": "/assets/icon128.png",
    "default_popup": "/options/options.html#popup"
  },
  "options_ui": {
    "page": "/options/options.html",
    "open_in_tab": true
  },
  "permissions": [
    "tabs",
    "storage"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "all_frames": true,
      "js": [
        "/content.js"
      ],
      "run_at": "document_start"
    }
  ],
  "background": {
    "scripts": [
      "/sw.js"
    ]
  },
  "commands": {
    "bigOptions": {
      "description": "Big options page"
    },
    "bigOptionsEditSite": {
      "description": "Big options page: edit site",
      "suggested_key": {
        "default": "Alt+Shift+C"
      }
    }
  }
}