Colour Contrast

As some may know, I'm in the process of redesigning this site, triggered in part because of the totally inaccessible manner in which it is displayed in IE7 (I would usually ignore that but it's one thing to not look too good, it's quite another to put the buttons in places that are unreachable). So, IE7 isn't all bad, it gave me something to investigate :). As part of the redesign I've been looking at ways of 'designing' my CSS files such that they can be easily edited and, as an added benefit, provides a means for user customization; in doing so I came across a problem.