Examine source code of Umschool extension

{
  "manifest_version": 2,
  "name": "Umschool extension",
  "version": "2.1.1",
  "description": "Small extension for Umschool website",
  "icons": {
    "32": "icons/32.png",
    "48": "icons/48.png",
    "128": "icons/128.png"
  },
  "browser_action": {
    "default_icon": "icons/128.png",
    "default_popup": "popup/popup.html",
    "default_title": "Umschool extemsion"
  },
  "content_scripts": [
    {
      "matches": [
        "*://umschool.net/*"
      ],
      "js": [
        "injected.js"
      ],
      "run_at": "document_start"
    }
  ],
  "permissions": [
    "tabs",
    "storage"
  ],
  "web_accessible_resources": [
    "dark.css"
  ]
}