Examine source code of ExtraMotivation

{
  "chrome_url_overrides": {
    "newtab": "index.html"
  },
  "content_security_policy": "script-src 'self'; object-src 'self'",
  "description": "Replace new tab page with Motivation. Once installed, enter your date of birth and then, every time you open a new tab, a live calculation of your age will appear. For test click: https://motivation-new-tab.github.io/",
  "icons": {
    "16": "icon-16.png",
    "48": "icon-48.png",
    "128": "icon-128.png"
  },
  "manifest_version": 2,
  "name": "ExtraMotivation",
  "version": "1.1.0",
  "browser_specific_settings": {
    "gecko": {
      "id": "ExtraMotivation@itsmohmans"
    }
  }
}