That fix has been added, though on its own it prevents you from including an image in the WYSIWYG editor. There's also an additional style property now that allows you to specifically style the WYSIWYG editor contents (which can set an image). If you don't do that, the values inherit from all sorts of places.
There isn't a way to put this in EXTRA.css, is there? Of course, if it's fixed in RC2 and that's down the pipeline, I suppose it's not a big deal anyway, but it may be worth knowing. Thanks!