Examine source code of z-index visualizer

{
  "description": "Visualize elements with z-index CSS property.",
  "manifest_version": 2,
  "name": "z-index visualizer",
  "version": "1.0",
  "homepage_url": "https://github.com/mdn/webextensions-examples/tree/master/open-my-page-button",
  "icons": {
    "32": "icons/icon.svg"
  },
  "page_action": {
    "default_icon": "icons/z-index-visualizer-off.svg",
    "default_title": "z-index visualizer"
  },
  "background": {
    "scripts": [
      "background_scripts/index.js"
    ]
  },
  "permissions": [
    "activeTab",
    "tabs"
  ],
  "browser_specific_settings": {
    "gecko": {
      "strict_min_version": "57.0"
    }
  }
}