To duplicate:

Choose Applications-->Preferences-->Advanced-->Window Appearance or 

Applications-->Preferences-->Advanced-->Sawfish-->Window Appearance

Click once on the arrow on the dropdown to select a new theme

Then, move to select another option from that list-- each time your mouse
moves over another theme, it will apply.  Applying themes in general is a
good idea-- i like to test them.  But it flashes by one after another just
moving to the one I want-- and sometimes when I'm just trying to click to
open the list! If I do it enough, rep will get all confused and hang sawfish.

Possible solutions:
Use a different widget to display themes list?
Delay a second before previewing?