I've just been stunned by a problem with Uschi vdR graffiti decals I can not cope with. Recently bought a set "10141 graffiti #2" (from their site: https://www.uschivdr.com/shopping-categories/shop-graffiti-marble-decals/) to find out that it is printed entirely on a white, non-transparent sheet. I have no idea what to do, because for the first time I see so complicated decals printed without a chance to cut them out. Help, maybe I'm missing some part of miraculous technique.

Sounds like they accidentally sent you, or you accidentally ordered, sheet 10142 (white), not 10141 (transparent).

They have two versions of the same sheet, one on a transparent backing and one on an opaque white background. Either you ordered the wrong set or they sent you the wrong set.

KL