That message says the XML doesn't haves any style to present it in a better way than a simple xml.This style can be a simple css like the css you use to style yout html webpages.
You should add this code to the header of the RSS file
<?xml version="1.0" ?><?xml-stylesheet type="text/css" href="http://your.site/rss.css" ?>And create an external css file with the filename which the xml-stylesheet tag previously added to your RSS will call.
rss { display: block; font-family: verdana, arial;}title { display: block; margin: 5px; padding: 2px; color: gray; border-bottom: 1px solid silver;}link { display: block; font-size: small; padding-left: 10px;}item { display: block; padding: 2px 30px 2px 30px;}docs { display: block; background-color: #ffffe6; margin: 20px; text-align: center; padding: 5px; color: #7f7f7f; border: 1px solid silver;}/* all hidden elements */language, lastBuildDate, ttl, guid, category, description, pubDate { display: none;}Feel free to modify the css ;-)