Updates the icon to use the icon named @icon_name from the
current icon theme as the base background image. If @icon_name
is %NULL, @self will go back using style information or default
theming for its background image.
If this method is called and a #GIcon was already set as
background for the icon, @icon_name will be used, i.e. the
last method called between gtk_numerable_icon_set_background_icon_name()
and gtk_numerable_icon_set_background_gicon() has always priority.
Updates the icon to use the icon named @icon_name from the current icon theme as the base background image. If @icon_name is %NULL, @self will go back using style information or default theming for its background image.
If this method is called and a #GIcon was already set as background for the icon, @icon_name will be used, i.e. the last method called between gtk_numerable_icon_set_background_icon_name() and gtk_numerable_icon_set_background_gicon() has always priority.