Examine source code of Center RFC

{
  "manifest_version": 2,
  "name": "Center RFC",
  "version": "1.3",
  "description": "Center RFC content on your monitor, cure neck pain :D",
  "icons": {
    "48": "icons/note-48.png",
    "96": "icons/note-96.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://datatracker.ietf.org/*",
        "https://www.ietf.org/rfc/*",
        "https://www.rfc-editor.org/rfc/*"
      ],
      "js": [
        "center_rfc.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{19745385-334e-490e-8a2d-b90e9a2c2cf9}"
    }
  }
}