I see this problem in longtime gtk applications
tried dark and light themes. have same problem

here preview :

inkscape:


gimp:


my settings:
https://i.hizliresim.com/vJ296A.png


also some applications showing black tooltip background in white

sample: chrome , firefox

Context of problem: sistem settings » color scheme » apply colors without qt-applications
I disabled this option, solved the problem

Painful. I think KDE leads, core devs and Blue Systems should also think about a full stack of Qt apps to complement the Plasma desktop.