Per the error warning I am getting, I need to switch to the default IPS theme.

Over the years they have been stacking up in my ACP.

How do I know which one is the latest, greatest and newest default theme?

 

 

None of them is “the” default theme. Essentially any new theme you create or existing stock theme that has no customizations matches the default. That’s the point of the error message. When there is a customization in a template, it cannot be upgraded and can likely fail. Restoring every template or creating a new one can resolve such issues. 

However, the shown error is probably caused by a third-party app (“dplus”). You will have to contact the author and get a new compatible version. In such a case, the error message can be misleading. The support tool will not help you in any way with that. If the the 3rd-party resource is incompatible with your IPS version, there is nothing you can do in the ACP. You need an update of the resource. 

 

P.S. Don’t keep all those themes around. You probably want to delete all those themes when they aren’t used.