GtkSourceView, modify background color in program code

[Stefan Salewski <mail ssalewski de>]

For GtkSourceView, it seems that setting background color in the
program code has no effect. I tried the traditional CSS stuff, but no
effect. When I do it for the Notebook widget containing the
GtkSourceView widget, it works. Seems that the StyleScheme always
overwrites the background color. (Well, I am not absolutely sure, I am
using a subclass of GtkSourceView, but I tried also giving the widget a
name with gtk_widget_set_name() and refer to that name from CSS code)