Examine source code of Hide Gmail Upgrade Button

{
  "manifest_version": 3,
  "name": "Hide Gmail Upgrade Button",
  "version": "1.0",
  "description": "Hides the \"Upgrade\" button in the Gmail header.",
  "browser_specific_settings": {
    "gecko": {
      "id": "gmail-no-upgrade@ian-mint",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "content_scripts": [
    {
      "matches": [
        "https://mail.google.com/*"
      ],
      "css": [
        "hide-upgrade.css"
      ],
      "run_at": "document_start"
    }
  ]
}