In Microsoft Dataverse it is possible to specify a theme to use for the organization/environment.
This is nice when a company brand can be implemented with color scheme and logo etc.

Another extremely common use of the themes is to be able to quickly distinguish between Prod/Test/Dev/etc environments in the same tenant, helping you not to make customizations in production, and other unwanted mistakes.

Since the move from the classic Solution Explorer to the Maker Experience, these themes are not reflected in any way.
Please bring themes back to the customizers! 😊