Examine source code of My Heading6 Checker

{
  "manifest_version": 2,
  "name": "H6 Checker",
  "version": "1.0",
  "description": "Highlight all H6 headings on a webpage.",
  "icons": {
    "48": "icons/icon.png"
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_icon": "icons/icon.png",
    "default_popup": "popup.html"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "css": [
        "styles.css"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{4e7c5aa4-f153-44aa-86f1-f89b8b79479d}"
    }
  }
}