Examine source code of Reading Helper Ruler

{
  "manifest_version": 2,
  "name": "Reading Helper Ruler",
  "version": "1.3",
  "description": "Reading helper with rectangle that follows the mouse",
  "browser_specific_settings": {
    "gecko": {
      "id": "extensions@gamartto.com"
    }
  },
  "permissions": [
    "activeTab",
    "storage",
    "<all_urls>"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content/content.js"
      ],
      "css": [
        "content/overlay.css"
      ],
      "run_at": "document_end"
    }
  ],
  "background": {
    "scripts": [
      "background/background.js"
    ],
    "persistent": false
  },
  "browser_action": {
    "default_popup": "popup/popup.html",
    "default_title": "Reading Helper",
    "default_icon": {
      "16": "icons/icon-16.png",
      "32": "icons/icon-32.png",
      "48": "icons/icon-48.png",
      "128": "icons/icon-128.png"
    }
  },
  "icons": {
    "16": "icons/icon-16.png",
    "32": "icons/icon-32.png",
    "48": "icons/icon-48.png",
    "128": "icons/icon-128.png"
  },
  "commands": {
    "toggle-helper": {
      "suggested_key": {
        "default": "Ctrl+Shift+L",
        "mac": "Command+Shift+L"
      },
      "description": "Toggle reading help"
    }
  }
}