Issue


You cannot close the Theme Builder: Discard & Close button does not help - it keeps loading forever:



Platforms

  • SharePoint 2013
  • SharePoint 2016

Reason


My Sites for your User Profile Service Application in your SharePoint Farm are not configured.


Solution


You need to configure My Sites to fix this issue.


Please, follow the steps in this solution article: How to Setup My Sites in SharePoint On-Premise.