Examine source code of Blackify

{
  "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": "Blackify",
  "version": "1.0",
  "homepage_url": "https://github.com/mdn/webextensions-examples/tree/master/borderify",
  "icons": {
    "48": "icons/border-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.soundcloud.com/*"
      ],
      "js": [
        "blackify.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{575f2e96-bbaa-4375-9f83-8932b19185e7}"
    }
  }
}