For something like that, you may want to check out
http://cleancss.com/ - copy the sheet's CSS into the box, set your selections as you wish, and let 'er rip. Quick, good, and pretty painless (aside from being an online tool rather than an integrated one).
I used CleanCSS on one book that had apparently been written with one sheet/chapter, then all the classes got mass-renamed so all the sheets could be crammed into one document. You'd end up with, for instance, .center, .center1, .center2, and so on through about .center29 - and they're all identical. CleanCSS combines those together, and then you can use Calibre's search/replace feature to reduce the references to all those different classes if you want.
That part can be tedious, but at least it's possible once everything's been combined.