Examine source code of Black Private Windows

{
  "manifest_version": 2,
  "name": "Black Private Windows",
  "description": "Windows in private mode have a black/grey theme.",
  "version": "1.0",
  "icons": {
    "48": "icon-48.png",
    "96": "icon-96.png"
  },
  "developer": {
    "name": "Tortuga"
  },
  "applications": {
    "gecko": {
      "id": "blackPrivateWindows@tortug.a",
      "strict_min_version": "67.0a1"
    }
  },
  "permissions": [
    "theme",
    "storage"
  ],
  "background": {
    "scripts": [
      "black.js"
    ]
  }
}