Since version 5.7, Qt has moved the qtstyle support from qtbase
to a separate repository.
For the user that means that he can't just simply export QT_STYLE_OVERRIDE=GTK+
to make Qt applications use the GTK widget style and theme.
To get the old behaviour back, you need to install qtstyleplugins.
Arch users can use this PKGBUILD.
Once you have it installed, don't forget to export QT_STYLE_OVERRIDE=gtk2
. Pay attention
to the gtk2 value. Before the split, the environment variable's value was GTK+, but
now you have to set it to gtk2.