Examine source code of Page Ruler Plus

{
  "manifest_version": 2,
  "name": "Page Ruler",
  "description": "Page Ruler: Draw rulers on web pages to check element alignment in Chrome.",
  "version": "2.2",
  "permissions": [
    "storage",
    "http://*/*",
    "https://*/*"
  ],
  "browser_action": {
    "default_popup": "index.html",
    "default_icon": "assets/default_icon.png"
  },
  "icons": {
    "16": "assets/icon-16.png",
    "32": "assets/icon-32.png",
    "48": "assets/icon-48.png",
    "128": "assets/icon-128.png"
  },
  "content_scripts": [
    {
      "js": [
        "src/scripts/utils.js",
        "src/scripts/helper.js",
        "src/scripts/app.js"
      ],
      "css": [
        "src/styles/inject.css"
      ],
      "matches": [
        "https://*/*",
        "http://*/*"
      ]
    }
  ],
  "web_accessible_resources": [
    "assets/*.png",
    "src/styles/inject.css"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{c09607a0-0bfd-4367-b759-4015c4fbdc5e}"
    }
  }
}