Comment # 15 on bug 932204 from Wolfgang Bauer

(In reply to Gerben Jansen from comment #14)
> I reported the same problem in bug 957441, but i can't set the style to
> Oxygen (it's not in the list).

Then install it.
The KF5 package is called oxygen5-style.

Or use KDE4's systemsettings as mentioned.

> I picked Fusion but that has the same problem. I'll just wait until this is
> resolved, don't want the bother with styles, etc.....

As I wrote here previously, Apper is a KDE4 application and the Fusion style
doesn't exist for KDE4. So yes, Fusion will not work.