For single posts, I see that category classes are included in the article tag, but not in the body tag. I believe that prevents us from hiding or displaying widgets based on the category, since the sidebar widgets appear in a different part of the HTML tree.
Are there any ways to use a non-inherited class in a different part of the CSS? Alternatively, are there any themes that place the category CSS class in the body tag rather than the article tag? (I've been looking with no success thus far.)