I suppose either dress theme is fine--bonus points for emo effort, but sweatpants are allowed if you're just too damn depressed to even bother. ;)

should people just be naturally depressed, or should they make a special effort, thereby turning it into a performance? I'd opt for the second interpretation.

Yeah, I was thinking totally hyperbolic theatrical depression, myself. The idea was to laugh in the face of depression by caricaturing it.