Howto make a Gtk.Buttonbox look like the path bar in Gtk.FileChooserDoalog?

[Moritz Renftle <rueckmeldung renftle net>]

Hi,

I recently noticed the nice appearance of the path bar widget in the
GtkFileChooserDialog:
http://imagepaste.nullnetwork.net/viewimage.php?id=4889
This look is also used for the top bar in the new GNOME system-settings.

I was wondering whether there is an easy way to turn a GtkButtonBox into
something with this look. At the moment, the ButtonBox in my
python-Gtk3-app looks like this:
http://imagepaste.nullnetwork.net/viewimage.php?id=4890

But I'd like to remove the vertical gap between the buttons in the
ButtonBox and have the nice rounded corners like in the path bar widget.
I tried to find the source code of the path bar but i failed reading the
c-code.

Is there a simple way to achieve this or would i have to implement my
own widget?

Thanks in advance for your help,
Moritz Renftle