Marble: Custom maps: How to create custom white global background for regional-only maps?

I am using Marble 25.12.2.

I have created some custom maps using WMS servers which cover only some regions.

If I am at the edge of the covered region, transparency seems to be returned.

This creates strange artifacts when scrolling or zooming near the edge of the defined area.

Example:



How can I avoid this?
E.g.: How can I create a custom white background for the whole world as “default”, and have the actual map overlayed on that?

Regards!

maybe @rahn ?