Examine source code of Custom Background

{
  "manifest_version": 2,
  "version": "1.0",
  "name": "Custom Background",
  "description": "Custom background for Firefox with beautiful images.",
  "icons": {
    "16": "img/icon16.png",
    "32": "img/icon32.png",
    "48": "img/icon48.png",
    "128": "img/icon128.png"
  },
  "chrome_url_overrides": {
    "newtab": "newtab.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{066567c4-c2d0-462e-9d12-e186dfc9106e}"
    }
  }
}