Adwaita: Propagate bg color to titlebar separator descendants

Adwaita: Propagate bg color to titlebar separator descendants

Gives the same background color to all separators descending from a
title bar than to its direct childrens.

This prevents separators which are in a titlebar but not direct children
from the widget with the titlebar style class from being almost
transparent and hence it prevent them from revealing the clear color of
the window's titlebar (black).

https://gitlab.gnome.org/GNOME/gtk/issues/1231