My favorite out of 8, Light!
Okay, these themes aren’t so amazing because they have bugs, but they also have a huge potential! It is a nice work from the author (Andrew -android-272- Pullins), but not really complete. The last updates came a few days ago.
The most exciting part is that we might see such a theme in Gimp GTK3 port, Gimp3. At least this what Andrew says:
I have been talking a lot with the Gimp devs and I might be working with them on a theme like this for GIMP 3.0
Gimp3 will come right after 2.10, and it could be quite usable right now if we put aside the fact that uses lots of CPU. Some bug I guess.
To try them (Gimp 2.8), download and place them inside Gimp themes directory.
~/.gimp/themes #Restart Gimp, if open!