I have been test driving 3.0. There is too much blue to my taste in the gray in the only dark theme offered (Arc Dark). I did not find any dark flat gray themes that completely work in 3.0 although they work in 2.8 and other distros with the XFCE DE. I found reading Spatry's Cup of Linux forum
http://cupoflinux.com/SBB/index.php/topic,4764.msg27694.html, that "Many of the themes that ship with XFCE are not gtk3.20 compliant yet so while they will theme your desktop, GTK3.20 applications will NOT use the older gtk styles and will not match. ... Searching online for themes which are gtk3.20 compliant will yield SOME results."
I found these themes work
http://www.ravefinity.com/p/vivacious-colors-gtk-theme.html 
