Gtk::Entry and modify_base

[Antonio Coralles <noche suapie reflex at>]

Is there a way to change the backround color of a Gtk::Entry when the 
user selects that entry with the mouse (and switching back to the old 
color when the curser leaves the entry) ? I've allready tried
modify_base(Gtk::// STATE_ACTIVE, ...)  and  
modify_base(Gtk:://STATE_SELECTED  , ...) without succes ...

Thanks,
               Antonio