Examine source code of DnD Beyond Fullscreen

{
  "manifest_version": 2,
  "name": "DnD Beyond Fullscreen",
  "version": "0.2",
  "description": "Adds a fullscreen button to the dnd beyond main menu",
  "homepage_url": "https://github.com/heikomat/dnd-beyond-fullscreen-firefox-addon",
  "content_scripts": [
    {
      "matches": [
        "*://*.dndbeyond.com/*"
      ],
      "js": [
        "add-fullscreen-buttons.js"
      ]
    }
  ]
}