This is mainly because we only just recently moved to being officially hosted by Stack Exchange. I'm assuming, but not for sure, that we just used the default blogging style used by ServerFault. I'm sure that in time that we can start moving over to a more personalized style for the blog and in fact have already started moving towards that way in small ways.
The Super User blog is hosted by Stack Exchange, but it is run by members of your community. The theme is the same, but the SU members that are administrators of the blog should be able to edit the css stylesheet as they see fit to make it SU's own.