Nice, calm and dark low contrast GTK+ theme.
XFCE Style seen in screenshot: Tyrex (unknown author)
"Soft and smooth and easy on the eyes. Matching metacity theme available and made to look its best with the Gnome Flat SVG icons.
Updated - a few bugs squashed for more consistency.
Yet another update - consistency improved throughout, much redundancy removed so as to speed up responsiveness.
Revised pretty substantially: lighter text fields that work better with evolution and help system, more rounded edges, and much improved responsiveness due to squashed bugs in notebook handling. Recursive inspiration from "StrongLooks" Clearlooks theme."
from: original description at art.gnome.org
see poptone's (=author) page (this guy also made a set of window borders names 'Tactile')