There is no theme for the 2004 calendar. The cover is a parody of a picture by Escher I think, but the rest is just made up of random gags. There was a post about this on alt.tv.futurama recently, and Ian Boothby (of Bongo Comics fame) had this to say:
I didn't do the cover but I did write the rest of them inside. The idea this
year for both this and the Simpsons calendar were just straight out gags. As
it turns out some of the calendar images are going to be made into Futurama
comic stories in the future as well.