Do not set treeview colors

A RHEL customer noticed a regression with overriding treeview colors from a GTK2 app when they updated from GNOME 3.14 to 3.22. @otte tracked this down to a change in gnome-themes-standard that regressed it. Would be nice to get this fix upstream as well.

From downstream bug report:

This seems to be happening because the updated Adwaita GTK2 theme decided to set the cell background color, which overrides the treeview's background color being used.

The code comments say that this was done for compatibility reasons with applications that look up these colors when drawing their own cell renderers. So I have no idea what the best way forward is in this case.