How to make Qt programs look good under Xfce?

This is not exactly an answer to your question but if you want a nice, consistent and unified look for both Qt and GTK programs I recommend the QtCurve style. Modern distributions should provide a package, in Ubuntu it's apt-get install qtcurve.

And yes, qtconfig is the program to use for configuring Qt-related stuff. In Ubuntu 16.04 you can install it via apt install qt4-qtconfig.

ok. sorted.

simply run qtconfig and select different look.

Excuse the slight self-promotion but I wrote a blog post and script to ensure QGtkEngine finds the correct GTK style: http://mikemcquaid.com/2011/12/01/make-qt-use-the-gtk-style-on-xfce-or-xubuntu/