Arc-Black-COLORS is a flat, dark-mode theme with transparent elements, for GTK, xfwm4, openbox-3, GNOME-Shell, and Cinnamon desktop environments. Black-on-black-on-black, with a single color, right where you want it.
Arc theme is 2019's most popular theme, as it has been for the past couple of years, according to major sources: (August 2, 2019) https://www.ubuntupit.com/top-15-best-gnome-shell-themes-gnome-desktop/.
The well-embraced standard Arc theme has always been available in the three variants of Arc, Arc-Dark, and Arc-Darker; but, at last, the measurably darkest dark-Arc has arrived on the GTK+ scene. Even at it dimmest, the earlier Arc variants have never quite satisfied those who need the lights turned down real low, but this darkest-of-all variant should fix all of that.
Special thanks to horst3180 at: https://github.com/horst3180/Arc-theme, for initially advancing Arc, and for inspiration. Further thanks to NicoHood at: https://github.com/NicoHood/arc-theme, for keeping Arc alive and great, more currently.
Manual Installation Is Easy:
Extract the "tar.xz" file into your "~/.themes/" folder - to install for current user only - or into the "/usr/share/themes/" folder - for the theme to be applied globally. 2) Use the GNOME Tweak Tool or an equivalent app to enable it for your desktop.
If installing manually, make sure to install the dependency: "Murrine theme engine" if you do not already have it; and update your GTK+packages if you have not.
Logging out and then logging back in may be necessary on some operating systems to fully implement theme.