Situation: you are on Workstation, Silverblue, AlmaLinux, CentOS Stream, some bootc image or any other distro with a GNOME Desktop.
You install a KDE app, may it be a Flatpak, in a Distrobox or as an RPM to the system.
It is always broken.
It seems to take some elements of the GTK/Adwaita theme into the Qt theme, but not all.
For example a dark GNOME theme, but with a white KDE Dolphin, white Text, white Icons.
This is unusable and I found no way to fix it.
What I tried in a Fedora Rawhide Distrobox, on top of HeliumOS (Alma 9 bootc):
Installing qt6ct
and changing themes there didnt do anything.
Installing plasma-systemsettings
resulted in an also unusable Window with no settings options.
Installing the lxqt config app resulted in a launch error.
How can I force KDE apps to just use their normal theme, and maybe switch light/dark?
The other way around, on KDE, Adwaita and GTK apps work perfectly fine. If I run
gsettings set org.gnome.desktop.interface icon-theme Adwaita
(On every launch, like with a systemd service) the GTK apps even use their normal Adwaita icons.