Hi @bjo
Same here on Ubuntu Mate 18.04.
I have noticed the same behavior on a RHEL 8.2 with gnome.
Developers created a internal theme manager, you can access it form KeePass menu.
It's the new default theme since KeePass 2.6.0 if I remember correctly.
If you want it to use your gtk theme, go to View, theme and choose classic and relaunch it.
Yes, switching to "classic" helps somehow. To get the look I also have on GTK, I had to use kvantum now and set it via QT_QPA_PLATFORMTHEME. Now everything looks fine.