Looks like this thread exists in two places but this is the more happenin' of them so I will repost here :)
An article was published recently on the topic of Innovative Design and Accessibility:
http://www.digital-web.com/articles/innovative_design_accessibility/The article doesn't address CMS or XOOPS for that matter but it's a worthy read nonethless.
Here are some other links on the topic:
Building Accessible Web Sites in Seven Steps
http://www.techsoup.org/howto/articlepage.cfm?ArticleId=381&topicid=13&cg=content&sg=feature
W3C Accessibility Quicktips Reference Guide
http://www.w3.org/WAI/References/QuickTips/
Web Content Accessibility Guidelines 1.0
http://www.w3.org/TR/WCAG/
5 First Steps for Designing Accessible Web Sites
http://www.ataccess.org/rresources/web/5firststeps.html
Accessibility Best practices
http://www.spingroup.com/htmdocs/resources/article.php?parent=5&catcode=57
The Sound of the Accessible Title Tag Separator
http://www.standards-schmandards.com/index.php?2004/11/06/6-the-sound-of-the-accessible-title-tag-separator
With regards to the creation of several user-selectable style sheets to enable font sizing - this is not necessary. With the use of em (instead of pt or px font sizes) the user can modify the font size with the browser function.
I'm currently redesigning my site (moving from phpnuke to xoops) and I can safely say that the templates themselves *can* be built for accessibility (using XHTML and CSS) with some decent hacking. Content may be another beast but the template is always the first step.