Examine source code of Dark Mode for Gmail V2

{
  "manifest_version": 2,
  "name": "Dark Mode for Gmail V2",
  "version": "1.3",
  "description": "Toggle and remember dark mode preference for Gmail.",
  "icons": {
    "64": "icons/logo_64.png"
  },
  "permissions": [
    "tabs",
    "storage",
    "https://mail.google.com/*"
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "content_scripts": [
    {
      "matches": [
        "https://mail.google.com/*"
      ],
      "css": [
        "styles.css"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_end"
    }
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": "icons/logo_64.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{d8acfe55-840e-4caf-b829-6c11f74226cb}"
    }
  }
}