Issue

You cannot create more than one theme using Color Theme Tool in ShortPoint Theme Builder for SharePoint 2019 environment:

• Once trying to save a new theme, the "Please wait.." message does not disappear:

• If you open any other option in Theme Builder and then go back to the Color Theme Tool, you will see your new theme saved, but you won't see the previous one in the themes list:

You will only be able to have one theme for Modern communication and teams sites on SharePoint 2019 environment.

Reason

This is a known limitation from Microsoft: custom modern themes are not supported for modern communication and team sites on SharePoint 2019 development platform. 

Note: To know more about this limitation, please check this article from Microsoft: SharePoint Server 2019 development platform.

Related articles:

• What permissions are required to create a SharePoint Company theme using ShortPoint Theme Builder?
• Known Issue: Modern Site Theme Does Not Change from Change the Look
• How to Publish a Theme in All Sites in a Site Collection, and Customize it on a Subsite