In progress
To create custom widgets and themes for ArcGIS Enterprise you will need to install Web AppBuilder (Developer Edition).
https://developers.arcgis.com/web-appbuilder/guide/getstarted.htm
https://developers.arcgis.com/web-appbuilder/guide/build-your-first-app.htm
https://developers.arcgis.com/web-appbuilder/guide/get-started.htm
Tips
When running Web AppBuilder, use your lower case machine name or FQDN instead of localhost or an IP address
Ensure CORS settings are clear in the target ArcGIS Enterprise, or that your machine name/FQDN above is white listed
Organization → Edit Settings → Security → Allow Origings