Comment # 5 on bug 969094 from Wolfgang Bauer

(In reply to Hrvoje Senjan from comment #4)
> I'll try to see with libqt5-qtct's maintaner what we can do about that.

I would propose to check the $XDG_CURRENT_DESKTOP variable and only set
QT_QPA_PLATFORMTHEME if it's not "KDE". (in libqt5-qtct's
/etc/profile.d/libqt5-qtct.*sh scripts)
Haven't tried that yet though.

Btw, isn't this a duplicate of bug#920792 ? ;-)