Examine source code of Darkify WhatsApp Web

{
  "name": "Darkify WhatsApp Web",
  "version": "1.0",
  "description": "Change WhatsApp web to dark theme by adding 'dark' class to body element",
  "manifest_version": 2,
  "permissions": [
    "activeTab"
  ],
  "content_scripts": [
    {
      "matches": [
        "https://web.whatsapp.com/*"
      ],
      "js": [
        "darkify.js"
      ]
    }
  ]
}