Examine source code of Messenger Dark Mode

{
  "manifest_version": 2,
  "name": "Messenger Dark Mode",
  "version": "1.0",
  "description": "Remove light-mode instances from the Messenger web app.",
  "permissions": [
    "activeTab"
  ],
  "content_scripts": [
    {
      "matches": [
        "*://www.messenger.com/*"
      ],
      "js": [
        "DarkMode.js"
      ]
    }
  ],
  "icons": {
    "48": "dark.png"
  },
  "browser_action": {
    "default_icon": {
      "48": "dark.png"
    },
    "default_title": "Messenger Dark Mode"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{6e087559-2f9e-49ab-9124-65f59e8b1b22}"
    }
  }
}