Examine source code of Umschool dark

{
  "manifest_version": 2,
  "name": "Umschool dark",
  "version": "1.1.0",
  "description": "Turns Umschool website into dark theme",
  "icons": {
    "32": "icons/32.png",
    "48": "icons/48.png",
    "128": "icons/128.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://umschool.net/*"
      ],
      "css": [
        "dark.css"
      ],
      "all_frames": true,
      "run_at": "document_start"
    }
  ],
  "permissions": [
    "*://umschool.net/*",
    "activeTab"
  ]
}