Examine source code of Toggle dark mode is

{
  "manifest_version": 2,
  "name": "Dark Mode Toggle",
  "version": "1.0",
  "description": "Toggle a dark background for websites",
  "permissions": [
    "tabs"
  ],
  "browser_action": {
    "default_icon": {
      "16": "icon.png"
    },
    "default_title": "Toggle Dark Mode"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{8b08b963-41e8-4217-8ef4-7d44abae5d7e}"
    }
  }
}