Examine source code of afore-css

{
  "manifest_version": 2,
  "name": "afore-default-css",
  "author": "khristopher pienda",
  "version": "1.0",
  "description": "in this extension, you can add the styles then you want to and that styles will be shown on every site was open on this browser",
  "content_scripts": [
    {
      "matches": [
        "*://*/*"
      ],
      "css": [
        "main.css"
      ],
      "run_at": "document_start"
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{e28ca986-6c8e-4f4e-b6c8-c091b45f0108}"
    }
  }
}