Polygons are defined by a set of rings, a closed lineString, so it is not surprising that you have the possibility of setting the stroke color and width. By defining a closed area, you may also set how this area has to be filled. The key element is <PolygonSymbolizer>; include it inside any rule you are defining for polygons. We will start with a fairly simple example.
Since you were a kid, you have been familiarized with the political maps of the world. Countries were rendered with brown boundaries and there were different colors for each country. Isn't this a wonderful example for your first polygon styling?
Note
You may wonder how many different colors you need to build a map where each adjacent country does not share the same color. The answer is not trivial; indeed, it is a surprisingly little number. Four different colors are enough for a map with any number of polygonal features. Take a look at http://en.wikipedia.org...