This is a modification of the "classic and confortable" Greybird by x-varlesh-x. It has a bit darker colors (to be more easy on the eyes), green accents and pressed buttons have the accent color. It is fairly skeuomorphic.
Fits well with my Skewaita GTK theme, although they are not really exactly the same. Also with Elementary icon theme (or its extension Elementary XFCE if you happen to use this desktop or other GTK-based ones).