The feed should be ok even if you see this message. You can check your feed for errors here http://validator.w3.org/feed/check.cgi

That "error" message occurs on thousands of perfectly well-functioning rss feeds and doesn't in any way indicate that there's anyhting wrong. Someone did explain it to me once. I've forgotten the details but it's a quirk in the way that rss was specified. AFAICR, the feeds worked best without this "style information", so everyone stopped adding it. It was too late to rewrite the whole thing, so you just get left with the error message.