How to use gtk_widget_modify_bg ???

[ramare <Olivier Ramare univ-lille1 fr>]

Dear all,

 Thanks for the pointer to gtk_widget_modify_bg but I'm
completely baffled as to how to put it to use ...
i'm rather happy to discover via google that lot's of
people seem to be in my situation, but I haven't been
able to get an inkling as to how .... Yes, I know,
that's despite howtos and faqs ... Maybe I'm particularly
thick but a complete example would help me (a window with
a label saying "Hello world!" with a choosable background
color without gong via rc files).
 Additionnal and connected question: how to know
which widget has a background field?
 Many thanks in advance!
 Cheers,
 Best,
      Olivier