Examine source code of Borderify

{
  "description": "Adds a solid red border to all webpages matching mozilla.org. See https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Examples#borderify",
  "manifest_version": 2,
  "name": "Borderify",
  "version": "1.0",
  "homepage_url": "https://github.com/mdn/webextensions-examples/tree/master/borderify",
  "icons": {
    "48": "icons/border-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.mozilla.org/*"
      ],
      "js": [
        "borderify.js"
      ]
    }
  ]
}