Source (link to git-repo or to original if based on someone elses unmodified work): https://github.com/Azarilh/Red-Breeze-Dark-GTK-2_3.2
It's a darker version of the original Breeze Dark with red instead of blue. It works on GTK 2.0, 3.0, 3.18 and 3.2. I tried it on Gnome and KDE Plasma, it works well.
See my profile for more Red Breeze Dark themes.
Cinereus (alternative GTK theme similar to this): https://www.opendesktop.org/p/1280788/