Examine source code of Focus Overlay

{
  "manifest_version": 2,
  "name": "Focus Overlay",
  "version": "1.0",
  "description": "Remove the mouse cursor and add a black opaque overlay to the page, creating a distraction-free environment for focused viewing or reading",
  "icons": {
    "32": "icons/plexiglass-32.png",
    "48": "icons/plexiglass-48.png"
  },
  "permissions": [
    "activeTab",
    "cookies",
    "<all_urls>"
  ],
  "browser_action": {
    "default_icon": "icons/plexiglass-32.png",
    "default_title": "Plexiglass Options",
    "default_popup": "popup/glass_options.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{94c5e0a8-26ad-4afd-9886-973828f0aab7}"
    }
  }
}