Examine source code of WhatsApp Web Customizer

{
  "manifest_version": 3,
  "name": "WhatsApp Web Customizer",
  "version": "1.4",
  "description": "Gray out muted chats on WhatsApp Web",
  "permissions": [
    "activeTab",
    "storage"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "zbirer@gmail.com"
    }
  },
  "content_scripts": [
    {
      "matches": [
        "https://web.whatsapp.com/*"
      ],
      "js": [
        "content.js"
      ],
      "css": [
        "styles.css"
      ]
    }
  ],
  "icons": {
    "24": "icon-24.png",
    "32": "icon-32.png"
  }
}