Examine source code of Focus Shade

{
  "manifest_version": 2,
  "name": "Focus Shade",
  "version": "0.3.29",
  "icons": {
    "16": "icons/focus-shade-16.png",
    "32": "icons/focus-shade-32.png",
    "48": "icons/focus-shade-48.png",
    "64": "icons/focus-shade-64.png",
    "96": "icons/focus-shade-96.png",
    "128": "icons/focus-shade-128.png"
  },
  "description": "Dim everything outside a selected page area to help you focus without removing page elements.",
  "permissions": [
    "activeTab",
    "tabs",
    "storage",
    "<all_urls>"
  ],
  "browser_action": {
    "default_title": "Focus Shade",
    "default_area": "navbar",
    "default_icon": {
      "16": "icons/focus-shade-16.png",
      "32": "icons/focus-shade-32.png",
      "48": "icons/focus-shade-48.png",
      "64": "icons/focus-shade-64.png",
      "96": "icons/focus-shade-96.png",
      "128": "icons/focus-shade-128.png"
    },
    "theme_icons": [
      {
        "light": "icons/focus-shade-16.png",
        "dark": "icons/focus-shade-16.png",
        "size": 16
      },
      {
        "light": "icons/focus-shade-32.png",
        "dark": "icons/focus-shade-32.png",
        "size": 32
      },
      {
        "light": "icons/focus-shade-64.png",
        "dark": "icons/focus-shade-64.png",
        "size": 64
      }
    ],
    "default_popup": "popup.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "focus-shade@local",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "focusOverlay.js"
      ],
      "run_at": "document_end"
    }
  ]
}