Blood and Custard. A theme for GNOME (GTK+ & Sawfish). Compared to other desktop environments there was no easy way to change the colours of GNOME apps. For even a simple theme, you had to specify colours for the widgets in a text file, maybe create a tarball and import it though GNOME control-center. It might be easier now, I don't know.
Anyway, there were any number of grey/dark themes and some great pixmap themes, but I wanted something that wasn't just eye candy, and still had a bit of colour.