Here are some changes that I had to make on my files, so I'm suggesting them as general changes.
/css_styles/default/default_style.css (removed Verdana as font type)
Verdana is a uncommon font and it messes up the proper display of the other fonts, just leave it as "Arial, Helvetica, sans-serif;" It looks better on both my pc and mac, ff, ie, safari.
RTE_javascript.asp (toolbar size changed from 600 to 100%)
non_RTE_javascript.asp (toolbar size changed from 600 to 100%)
/includes/message_forum_inc.asp (width changed from 600 to 100%, width changed from 596px to 100%)
/includes/quick_reply_form_inc.asp (width changed from 600 to 100%, width changed from 596px to 100%)
The last four files created a formating issue for me as I put the forum into an existing table / website. The forum kept breaking out of the presized parent table and messing up the formating. By seting them to a percentage, they properly autosized to fit my site.