Gosh... Very big shame on me..... It does work that way! Thank you VERY much, Hansmex!

Remains only the problem of importing a new floor texture as a JPEG image. When clicking on the room surface in the 2D-plan, one should be able to change the color or texture of floor and ceiling, and if needed add new ones.
I can't do that. Is it also a stupid "click-error", do you think?