I believe the GNOME theme manager program is part of the "control
center" installed with GNOME (at least on Red Hat 6.0 desktops).
It should be in the "settings" or "preferences" (I don't remember
which) menu on your GNOME panel.
There should be a tab within the Control Center for your desktop
settings, and it will let you pick a theme. If these themes affects
the display of GTK widgets, AbiWord should honor them.
-- Shaw Terwilliger