Examine source code of Font Sizes

{
  "manifest_version": 2,
  "name": "Font Sizes",
  "version": "1.0",
  "permissions": [
    "<all_urls>"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "font-sizes@example.com"
    }
  }
}