I use qtcurve.
I have a little problem with how prompts are displayed on gtk application. I have adwaita set for gtk apps. But it should be managed by qtcurve I think.
Here is example of Cherrytree
https://i.imgur.com/7GxEpIf.png

White font is almost invisivible. I want it to look as qt applications, with breeze dark look which I have set for Plasma.
Changing settings for gtk to dark theme is not an option because it makes using other program difficult. But again, I think it should be somewhere is qtcurve settings, but there are so many of them that I don't know which it might be.

I'm a little confused, you mention setting the gtk theme to qtcurve and to adwaita. What exactly do you have selected for the GTK2 and GTK3 themes under (KDE) System Settings -> Application Appearance -> Gnome (GTK)?

I've had mixed results with different gtk apps. I've currently got both gtk2 and gtk3 set to adwaita-dark, with Prefer Dark themes checked. Most of my gtk(3) apps are cooperating with the dark theme, including Firefox. Simon Tatham's puzzles, which use GTK2, still do white on white (light gray) menus

I posted a bug report:
https://bugs.kde.org/show_bug.cgi?id=398021

The problem is that qtcurve always gets back to (Current) Preset.

If anyone knows how to edit (Current) Preset let me know.