Citation:
modify_bg() only affects widgets that have an associated gtk.gdk.Window. Widgets that do not have an associated window include gtk.Arrow, gtk.Bin, gtk.Box, gtk.Button, gtk.CheckButton, gtk.Fixed, gtk.Image, gtk.Label, gtk.MenuItem, gtk.Notebook, gtk.Paned, gtk.RadioButton, gtk.Range, gtk.ScrolledWindow, gtk.Separator, gtk.Table, gtk.Toolbar, gtk.AspectFrame, gtk.Frame, gtk.VBox, gtk.HBox, gtk.VSeparator, gtk.HSeparator. These widgets can be added to a gtk.EventBox to overcome this limitation.