"Graphical application for generating different color variations of a Oomox (Numix-based), Materia and Arc themes (GTK2, GTK3), Gnome-Colors and Archdroid icons and terminal" (ibd., 2018-10-21)
"A flat dark GTK-theme with ergonomic contrasts. It supports Gnome, Cinnamon, Xfce4, Mate and Openbox."
Cf. https://github.com/WernerFP/Shades-of-gray-theme