Examine source code of Heading 4 Checker

{
  "manifest_version": 2,
  "name": "H4 Checker",
  "version": "1.0",
  "description": "Highlight all H4 headings on a webpage",
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_popup": "popup.html",
    "default_icon": {
      "48": "icon.png"
    }
  },
  "icons": {
    "48": "icon.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "css": [
        "styles.css"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{2802eb13-122d-40f9-844b-8034f72b0b76}"
    }
  }
}