Examine source code of Dark Messenger

{
  "manifest_version": 2,
  "name": "Dark Messenger",
  "version": "1.0",
  "description": "Darkify Messenger",
  "icons": {
    "48": "icons/dark_messenger.png"
  },
  "content_scripts": [
    {
      "matches": [
        "https://www.messenger.com/*"
      ],
      "css": [
        "styles.css"
      ]
    }
  ]
}