Examine source code of GreyKeep

{
  "manifest_version": 2,
  "name": "GreyKeep",
  "version": "2.1resigned1",
  "description": "Sets background in Google Keep to light grey",
  "icons": {
    "48": "icons/greykeep-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://keep.google.com/*"
      ],
      "js": [
        "greykeep.js"
      ]
    }
  ],
  "applications": {
    "gecko": {
      "id": "madeupmail@gmail.com"
    }
  }
}