Wrong definition of Bartik's fieldset description in css

While building a fieldset with description, I saw that it has no any css style applied. Looing the style.css file I noticed that "fieldset .definition" should be renamed to "fieldset .fieldset-definition". I've also moved the definition just below the rest of fieldset stylings in the patch.

Finally, even when applied, I found that the bottom margin sould be increased a little bit, because the description finishes just where the first form element starts, very difficult to see from usability point of view (see the screenshot included), however I'm not sure about how much margin to put between description and the field, so I've included 20px as starting point.