please, follow the styles tutorial STEP by STEP.
in short, you need to first list all signs by hitting that button like described in that tutorial, than select those letters and asign the proper font to them until they are displayed properly. from then on this is the font to which a sign will be changed.
if you are truly keen you can edit your pref file with TextWrangler, it is just plain xml... but don't be too keen, or you might forever loose it
