Examine source code of Grayscale Mode

{
  "manifest_version": 2,
  "name": "Grayscale Mode",
  "version": "1.0.0",
  "description": "Grayscale page",
  "icons": {
    "48": "icons/icon.svg"
  },
  "permissions": [],
  "browser_specific_settings": {
    "gecko": {
      "id": "grayscalemode-03883a7b@local",
      "data_collection_permissions": {
        "required": [
          "none"
        ],
        "optional": []
      }
    }
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ]
}