The stylesheet used for http://static.springframework.org/spring-security/site/ doesn't set the background/foreground colours on the page as a whole, only on individual elements (and a background image with a transparent background on #contentBox). This means that the browser defaults show through, which don't necessarily mix well with the green background they're on on the sidebar, or the off-white they appear on in the main area.
Try setting the browser defaults to white text on a black background (a high contrast setup) and you'll see what I mean - the site is illegible. You need to make the defaults explicit in maven-base.css:
... and the site will read just fine.
NB while WCAG does suggest that /not/ setting background and foreground colours is a way to achieve accessibility ( http://www.w3.org/TR/2008/NOTE-WCAG20-TECHS-20081211/G148 ) that's not whats happened here - colours are specified for links, for the background of sidebars etc, and there is a coloured background image in the main content area.