The package gnome-themes installs the GTK2 part of the engines, but not the GTK one (i.e. smooth engine). I've noticed the gtk-engines-smooth package does detect if GTK or GTK2 are installed and then decides which parts to install. I think this funcionality should be ported to the gnome-themes package. As gnome-themes is a dependency of gnome, and gtk-engines-smooth is a blocker for gnome-themes, there isn't a clean way of having gnome and the GTK engines installed togheter (AFAIK). Could be a solution making gnome-themes a metapackage that installs the corresponding engines?
Going to mark this as upstream since there is nothing we can do about this for the current 2.8.x releases. All engines have been moved from gnome-themes to gtk-engines for the 2.10 release, so that should solve this problem.
