Examine source code of Graythumb

{
  "manifest_version": 2,
  "name": "graythumb",
  "version": "1.1.1",
  "description": "Turn Youtube thumbnails into grayscale images",
  "icons": {
    "48": "icons/graythumb-48.png",
    "96": "icons/graythumb-96.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.youtube.com/*"
      ],
      "js": [
        "graythumb.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "__graythumb@diegosandoval.net"
    }
  }
}