BSM Reborn is an unofficial continuation of the old BSM Simple theme. I decided to reboot this theme, because it didn't have GTK3 support and the original author has yet to update it. I didn't create this theme from scratch. I've combined elements from the original BSM Simple theme, as well as elements from Ambiant MATE to provide missing pieces (with color schemes changed from green to gray). The result is as close to a continuation of the BSM theme as I could get without being an artist myself. I can't take credit for creating this theme since I just used elements from other themes, but it took a lot of hard work editing the components to make everything look good. This theme has been tested on Ubuntu MATE 16.04. It may work on other distributions, but I don't promise it.
To install, extract the "BSM Reborn" directory in the archive to either ~/.themes (create the ~/.themes directory if it doesn't exist) or copy it to /usr/share/themes/ for a system-wide installation.
The theme is pictured with the "Moka Minimal" icon theme. I think that icon theme goes really great with it.