Examine source code of Background-theme-pretty

{
  "manifest_version": 2,
  "name": "Borderify",
  "version": "1.0",
  "description": "Adds a background image to all webpages matching google.com",
  "icons": {
    "48": "icons/border-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.google.com/"
      ],
      "js": [
        "borderify.js"
      ]
    }
  ]
}