You can use any GTK 3 and GTK 2 Theme in XFCE,LXDE and i believe in other Desktop envoronments based on GTK, as of XFCE 4.8 the XFCE theme engine only supports GTK 2, in XFCE 4.10 GTK 3 support has been added, so when XFCE gets distributed to all distros you'll only need a GTK 3 Theme instead of both 2 & 3 as you do now. So for example I use Nautilus, so I need A GTK 3 Theme but at the same Time a GTK 2 Theme to theme the rest of the Environment, such as the XFCE settings, Panel Settings, etc.
I have Ubuntu 12.04 (much perfer Arch though...) and I installed it, apt-get remove purge unity, then install elementaryOS Luna form PPa, and it works very good! ... but I love XFCE and would much rather have that. So yeah, just wondering about if i will have to have GTK2 themes and such installed... looks like the answer is no!
Thinking of seeing if a PPA for 4.10 exists (yet) and installing it, but not installing thunar a show i really like Marlin.
I have yet to see a PPA for 4.10, you can Install the metapackage for XFCE, and then remove thunar, though you'll have problems with xfdesktop if you do, for example you will no longer be able to access the desktop preferences from right click on the desktop, so you would have to kill the process and that would leave you with the Gnome Desktop but with the rest f XFCE still running.