Examine source code of Reading Ruler

{
  "manifest_version": 2,
  "name": "Reading Ruler",
  "version": "1.0",
  "description": "Adds a reading ruler to help follow lines while reading.",
  "permissions": [
    "activeTab"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "ruler.js"
      ],
      "css": [
        "ruler.css"
      ]
    }
  ],
  "icons": {
    "48": "icon.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{497db1ff-ce62-44bc-93a1-270c3b34ddac}"
    }
  }
}