git clone https://github.com/GNOME/gtk/
git checkout 34de998c389f750c749883290a3d3c2212163c44 (maybe not necessary)

apply patch diff.txt

then generate gtk-contained-dark.css with sassc:
cd gtk/gtk/theme/Adwaita/
export LD_LIBRARY_PATH=/home/sed/.config/gtk-3.0/usr/lib/x86_64-linux-gnu
~/.config/gtk-3.0/usr/bin/sassc gtk-contained-dark.scss gtk-contained-dark.css

in ~/.config/gtk-3.0, link to css and assets:
/home/sed/.config/gtk-3.0> ls -alrct
lrwxrwxrwx  1 sed sed     52 Jul 15 22:58 gtk.css -> /home/sed/.themes/gtk/gtk/theme/Adwaita/gtk-dark.css
lrwxrwxrwx  1 sed sed     62 Jul 15 22:59 gtk-contained-dark.css -> /home/sed/.themes/gtk/gtk/theme/Adwaita/gtk-contained-dark.css
lrwxrwxrwx  1 sed sed     46 Jul 15 23:07 assets -> /home/sed/.themes/gtk/gtk/theme/Adwaita/assets