GTK+ 2.8.17 and background colours

[Francesco Montorsi <f18m_cpp217828 yahoo it>]

Hi,
     I'm struggling with my GTK+ (2.8.17 - coming with ubuntu dapper) to
be able to set the background colour of a button.

I've googled and searched the archives of this mailing list and I have
found exactly same questions with attached test programs.

I'm attaching two short, simple test programs which *should* be able to
set the background colour of a button to a different colour from the
standard one.

Unfortunately running them I don't see any difference with a standard
GtkButton: i.e. the calls to gtk_widget_modify_bg and
gtk_widget_modify_fg seems not to have any effect !

Could anyone more expert try to compile those files and run them to see
if they get standard colours or not ?
(to compile them I usually place them in the gtk\examples\helloworld
folder and rename them to helloworld.c ;))

This seems like a GTK+ bug to me...

Thanks a lot,
Francesco