Comment # 23 on bug 920792 from Wolfgang Bauer

(In reply to Simon Lees from comment #22)
> I don't see any reason to keep this open.

But this is a completely different problem, totally unrelated to any GTK style.

Short summary:
Installation of libqt5-qtct creates a script in /etc/profile.d/ with "export
QT_QPA_PLATFORMTHEME=qtct".

This forces the qtct platform plugin to be used, even if you login to Plasma5.
Because of that, KDE's settings are not respected, icons may be missing
(because they are not in the used fallback icon theme), and maybe other
problems.