Examine source code of Duolingo Dark Mode

{
  "manifest_version": 2,
  "name": "Duolingo Dark Mode",
  "version": "0.2",
  "description": "Dark mode for Duolingo",
  "content_scripts": [
    {
      "css": [
        "styles.css"
      ],
      "js": [
        "content.js"
      ],
      "matches": [
        "https://www.duolingo.com/*",
        "https://www-internal.duolingo.com/*",
        "https://join.duolingo.com/*"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{ea6818e0-8ffc-4145-ac28-f048a9425d4d}"
    }
  }
}