Examine source code of Enhance YouTube

{
  "manifest_version": 2,
  "name": "Magic Actions for YouTube (Demo)",
  "version": "1.0.0",
  "description": "Adds handy controls for YouTube: mouse-wheel volume, theater mode, auto-HD, screenshot, PiP, and more.",
  "permissions": [
    "activeTab",
    "tabs",
    "storage",
    "<all_urls>"
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "browser_action": {
    "default_title": "Magic Actions for YouTube",
    "default_popup": "popup.html"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.youtube.com/*"
      ],
      "js": [
        "content-script.js"
      ],
      "run_at": "document_end"
    }
  ],
  "applications": {
    "gecko": {
      "id": "magicactions-youtube@example.com"
    }
  }
}