Nope, I don't believe that "racism" (etc.) is any more objectively defined a category than "theft", and I don't think there's any particular definition of it we should all agree on. I agree with you that disagreements about category membership can arise even when there's agreement on facts, both with respect to "racism" (etc.) and more or less every other human category.

(This is not surprising, given that the way human brains categorize percepts and concepts maps very imperfectly to how we imagine definitions working; the whole idea of categories having definitions is a very poor approximation of what's going on. But that's a digression.)

If anything I've said implies that "racism" or any other category has an objective definition in the sense you seem to mean here, I've completely missed that implication and am likely very confused. If you feel inclined to unpack what it was I said that implies that, I'd be appreciative.