Examine source code of Font Deugly.

{
  "manifest_version": 2,
  "name": "Font Deugly",
  "description": "Deuglify Fonts on Social Media Sites",
  "version": "2.1.9",
  "icons": {
    "512": "icon.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "fontdeugly2@djwong.org"
    }
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.git.kernel.org/*",
        "*://git.kernel.org/*",
        "*://*.git.infradead.org/*",
        "*://git.infradead.org/*",
        "*://knowablemagazine.org/*",
        "*://*.marc.info/*",
        "*://*.mail-archive.com/*"
      ],
      "js": [
        "font_deugly.js"
      ],
      "run_at": "document_end"
    }
  ]
}