Examine source code of Private GitHub Header

{
  "manifest_version": 2,
  "name": "private-github-header",
  "version": "0.1.1",
  "content_scripts": [
    {
      "matches": [
        "https://github.com/*"
      ],
      "js": [
        "private-github-header.js"
      ]
    }
  ]
}