I have tried to install Kubuntu 15.10 and Ubuntu using the netinstaller, everything is working fine.
The problem is, KDE uses only one Theme, which is the GTK+ theme. Meaning I can't change them using the System settings. I checked environment variables.
QT_OVERRIDE_STYLE (or whatever it's called) is not set. However systemsettings5 saves its changes to
~/.config/Trolltech.conf, but it looks like the changes aren't respected.
I have even tried to use the PPA with the "staging" version of Plasma. Unfortunately that doesn't work either. The problem also occurs on new users.
That's why I think it could have something to do with the default settings Ubuntu uses to make KDE apps look good on Unity.