Examine source code of The Reading Ruler

{
  "manifest_version": 3,
  "name": "The Reading Ruler",
  "version": "2.1.0",
  "description": "A ruler that overlays a webpage to help you read & retain information. Helpful for people who are neurodivergent.",
  "author": "webDevLiam",
  "permissions": [
    "storage"
  ],
  "action": {
    "default_popup": "popup.html",
    "default_title": "The Reading Ruler"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "assets/scripts/reading_ruler.js"
      ]
    }
  ],
  "icons": {
    "16": "assets/imgs/16icon.png",
    "32": "assets/imgs/32icon.png",
    "48": "assets/imgs/48icon.png",
    "128": "assets/imgs/128icon.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "liamoconnor87@gmail.com"
    }
  },
  "commands": {
    "toggle-overlay": {
      "suggested_key": {
        "default": "Ctrl+Shift+U",
        "mac": "Command+Shift+U"
      },
      "description": "Toggle the overlay on/off"
    }
  },
  "background": {
    "scripts": [
      "assets/scripts/background.js"
    ]
  }
}