Fix the style schemes when used with a dark gtk+ theme

This affects the classic and tango style schemes.

The problem was that some colors were taken from the gtk theme.
I think we don't have the choice, and have to hard code the colors. They
come from:

https://git.gnome.org/browse/gnome-themes-standard/tree/themes/Adwaita/gtk-3.0/gtk-main.css#n1