Examine source code of Square YT Player

{
  "manifest_version": 2,
  "name": "Square YT Player",
  "version": "1.0",
  "description": "Removes rounded edges off of the youtube video player",
  "icons": {
    "48": "logo.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.youtube.com/*"
      ],
      "css": [
        "square.css"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{bc2f1a24-5843-4379-9b0a-357011296a45}"
    }
  }
}