Hello!

Is there a way to make color settings from systemsettings5 affect gtk+ applications?

In systemsettings5 "appearance" - "color" - "colors" - "window background" is changed, but it does not affect gtk+ applications, such as firefox (tab area) and filezilla (menu panel area), see screentshot.

in systemsettings -> Application Style there should be a Gnome Application Style (GTK) module, in openSUSE it is the package kde-gtk-config5

kde-gtk-config5 - KCM Module to Configure GTK2 and GTK3 Applications Appearance Under KDE

kde-gtk-config is a KCM module to configure GTK2 and GTK3 applications appearance under KDE.
Among its many features, it lets you:
- Choose which theme is used for GTK2 and GTK3 applications.
- Tweak some GTK applications behaviour.
- Select what icon theme to use in GTK applications.
- Select GTK applications default fonts.
- Easily browse and install new GTK2 and GTK3 themes.

there are also some gtk2 and gkt3 breeze and oxygen theme packages that are probably used, also there is a long thread "how to integrate Firefox into KDE" viewtopic.php?f=14&t=17786&hilit=theme+firefox

I am using Arch Linux, and for the purpure of integration there are breeze-gtk and kde-gtk-config packages, they are installed. I have 'breeze' theme selected in the 'application style' - 'gtk' but it appears to not to use color theme (or use the one in the original breeze theme regardless of the changes I made in 'color' section of systemsettings).

And the question is not only about firefox (I know it may have a lot of specific things), but about, say, filezilla also.

you may need to use the Gnome config app to change those, don't use so not sure

A hickup with the GTK+ 3.20:

Bug 361066 - [Feature Request] GTK+ 3.20 support : https://bugs.kde.org/show_bug.cgi?id=361066