When you want to ask for the theme to someone who is dressing up as something, which is the correct English? "what's the theme?","what's the theme of?" or anything else more natural expressions??
I have certainly heard and used "theme" in this way (ie, a theme party, aka a costume party). A Google search brings up many hits for this.
The correct way if you are asking about a party that has already been would be "What was the theme"? Or, for an upcoming party, "What's the theme?" is fine.
Of course, not all parties are theme parties, so then "Is there a theme?" is best.
Using "costume party" is a little different -- you would still say "What is the theme?" and not "What is the costume?" (unless you mean to ask who/what someone is dressed up as) but "Is/was it a costume party?" is fine.