Examine source code of YouTubeDark

{
  "manifest_version": 2,
  "name": "YouTubeDark",
  "version": "1.3",
  "description": "Auto sets youtube to dark mode.",
  "icons": {
    "48": "icons/youtube_48_black.png"
  },
  "permissions": [
    "activeTab"
  ],
  "content_scripts": [
    {
      "matches": [
        "*://*.youtube.com/*"
      ],
      "js": [
        "youtubedark.js"
      ]
    }
  ]
}