Examine source code of domain.cls

{
  "manifest_version": 2,
  "name": "domain.cls",
  "version": "1.6",
  "description": "Add 'domain' to body class",
  "icons": {
    "48": "darkdev_48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "add_domain_cls.js"
      ]
    }
  ]
}