Source (link to git-repo or to original if based on someone elses unmodified work): https://github.com/solsticeman/Arc-Dark-Theme-Collection
These themes are Arc-Dark, the darkest variant of the arc theme, which is a flat theme with transparent elements for GTK 3, GTK 2 and GNOME Shell in support of GTK 3 and GTK 2 based desktop environments such as GNOME, Unity, Pantheon, Xfce, MATE, Cinnamon (>=3.4), Budgie Desktop (10.4 for GTK+3.22) etc.
Tested with LM 20.0 Cinnamon and XFCE, and with Ubuntu 20.04 LTS.
NOTE: Since these GTK themes also work in XFCE, the XFCE Arc-Dark Theme Collection VOLUME I is no longer published.
BONUS: Arc-Shark-Red, not pictured, added to mix! (uses Arc-Shark icons)
Beautiful, flat Suru++ monochromatic icons by Sam Hewitt complement the themes.