Examine source code of Messenger Dark

{
  "manifest_version": 2,
  "name": "Messenger Darkify",
  "version": "1.15resigned1",
  "description": "Gives a dark theme to the Facebook messenger website.",
  "icons": {
    "48": "icons/donut_48dp.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.messenger.com/*"
      ],
      "css": [
        "darkify.css"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{fd516298-98b1-4f62-96a1-4633537fee2d}"
    }
  }
}