XFCE is a GTK2 environment, however a lot can be done to improve GTK3 apps on XFCE.
I prefer to use Clearlooks as my GTK2 theme and use the GNOME icon set. On Debian,
apt-get install gnome-icon-theme gnome-icon-theme-extras gnome-icon-theme-symbolic clearlooks-phenix-theme and use the XFCE Appearances to change your theme to Clearlooks-Phenix and your icons to GNOME.
If you’re using an XFCE GTK engine theme instead, install
gtk3-engines-xfce instead of
clearlooks-phenix-theme, and there also is a
gtk3-engines-oxygen which provides a native look-alike of KDE4’s Oxygen theme.
You probably should restart your X session after fiddling around with this to fix apps that don’t change themes at runtime properly.