Examine source code of Force Focus Outline

{
  "manifest_version": 2,
  "name": "Force Focus Outline",
  "author": "Leni Marvaud",
  "icons": {
    "48": "logo.png",
    "96": "logo@2x.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{a7ea7e12-cfec-46d7-acf9-399b12fd9f06}"
    }
  },
  "version": "1.0.0",
  "description": "Forces an outline on all focus-visible elements on all web pages.",
  "permissions": [
    "activeTab"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "css": [
        "force-focus-outline.css"
      ]
    }
  ]
}