Examine source code of Duolingo Sound Controls

{
  "name": "Duolingo Sound Controls",
  "version": "1.7.1",
  "description": "Provides fine-grained sound controls for Duolingo.",
  "permissions": [
    "activeTab",
    "storage",
    "https://*.duolingo.com/"
  ],
  "background": {
    "scripts": [
      "src/background.js"
    ],
    "persistent": true
  },
  "content_scripts": [
    {
      "matches": [
        "https://*.duolingo.com/*"
      ],
      "js": [
        "src/content.js"
      ],
      "run_at": "document_start"
    }
  ],
  "page_action": {
    "browser_style": false,
    "default_popup": "pages/popup.html",
    "default_icon": "icons/icon_48.png"
  },
  "web_accessible_resources": [
    "assets/css/ui.css",
    "src/observer.js"
  ],
  "icons": {
    "16": "icons/icon_16.png",
    "48": "icons/icon_48.png",
    "128": "icons/icon_128.png",
    "256": "icons/icon_256.png"
  },
  "manifest_version": 2,
  "browser_specific_settings": {
    "gecko": {
      "id": "{3f28559e-aa3b-44cb-bf75-1c42bca8e448}"
    }
  }
}