GnomishDark is a dark GTK 3 theme for Ubuntu 11.10 Oneiric Ocelot that is compatible with both Unity and Gnome Shell.
The theme is based on Adwaita and can look attractive if you run it side by side with one of these icon themes listed here.
Via the terminal run this sequence of commands:
wget -O GnomishDark.tar.bz2 http://goo.gl/Z0PT3
tar jxvf GnomishDark.tar.bz2 -C ~/.themes
To enable GnomishDark, run these two commands from the terminal:
gsettings set org.gnome.desktop.interface gtk-theme 'GnomishDark'
gconftool-2 --set --type string /apps/metacity/general/theme 'GnomishDark'
Or do it graphically using Gnome Tweak Tool.