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.