Examine source code of Grayscale Filter

{
  "description": "Adds a grayscale filter to all webpages",
  "manifest_version": 2,
  "name": "Grayscale Filter",
  "version": "1.0",
  "icons": {
    "48": "icons/icon.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "grayscale.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{5d1a00e0-bdaa-4a47-a5f6-95bfa65a50ad}"
    }
  }
}