The default Ubuntu theme for new users in Ubuntu is the Ambance theme. I don't like it and want something completely different. That's easily done this way:
Place the gnome config files into a folder called /etc/skel, any new user you create will have those files in there home folder.
thought I'd add the folder you want to put in /etc/skel is .gconf
And you should make your installed theme available in /usr/share/themes and icon theme in /usr/share/icons.
This should work.