Sometimes I get the error that seems to be described in this thread (see this post for an image). However, despite it being an error, I actually enjoy the theme (or lack thereof) with it's carved buttons, no rounded corners and clear boundaries between elements. Maybe it just feels nostalgic, but anyway -- I want to know how to keep that theme (non-theme?) permanently.
Btw, it doesn't seem to be the Clearlooks theme, which according to what I read is the default Gnome theme, so I'm inclined to believe that it is Gnome unthemed.