How to change the GtkNotebook background color

Most of the widgets in Glade-3 have grey backgrounds. GtkNotebook however has a white background for some reason, so when widgets are added to it, it looks ugly. I've been trying allsorts of things to modify the background color of GtkNotebook from white to grey without avail.
Does anyone here have a simple way of doing this?

I am afraid but I think the only way is to do that with CSS as the background-controlling functions are all deprecated in Gtk3.

Here you see how this is made in Gmoccapy: github.com/hansu/linuxcnc/blob/d4c703aeb...moccapy.py#L145-L168

Only if you are using Gtk2 / Linuxcnc 2.8 you can maybe use modify_bg() or similar.