I do significant restyling as myself, but not as Anonymous Monk, obviously, and afaict it looks fine as Anonymous Monk for me. Does it not for you?
I have an alter ego which does not have any custom css. With this user, I cycled through the themes and never saw any problem with the headers being huge.
My complaint was about the headings being huge. The table is significantly wider due to that.
I don't know why you are getting that. I have to suppose you have some kind of unfortunate combination of styling that makes it come out that way.
Oh, wait. Do you have "Large Code Font" enabled? :-)