Examine source code of Spotlight Darkmode

{
  "manifest_version": 2,
  "name": "Spotlight Darkmode",
  "version": "1.0",
  "description": "Creates a black overlay with a spotlight to view contents underneath.",
  "web_accessible_resources": [
    "content_scripts/spotlight.js",
    "content_scripts/spotlight.css"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content_scripts/spotlight.js"
      ],
      "css": [
        "content_scripts/spotlight.css"
      ],
      "run_at": "document_end"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{3c845367-b89f-477d-93b6-e0d6db4143b3}"
    }
  }
}