I'm entirely happy with the way PM looks. The text is the right size and everything is super easy on the eyes. But I also have Perl-Blue theme set and a permanent login cookie.
Occasionally I clear my browser cookies and then I'm faced with the default theme of an anonymous user. At times like this I'm reminded of how much I like the Perl-Blue theme.
I agree that the default PM theme is a shock to the web surfing experience. I don't think it puts the site's best foot forward and I believe (with no facts to back me up) that many other monks have also changed their theme settings to avoid it. And because of that we just don't care what the rest of the world sees, because PM works for us.
I would suggest changing the default theme is the simple answer to most of the 'why is this site so ugly' comments that pop up from time to time. But to 'what theme' is as subjective as anything else. Perhaps there are some stats as to the most used theme by PM users?