How absolutely retro, Rocky Horror!!! I remember going to see it when I was in my early 20’s. During subsequent showings, people often go in costume, which might be something guests will want to do for your daughter’s party. Send out a list of potential characters and let folks pick which one they want to be. As for decorations, have the movie playing in one room, set up your main rooms in a “castle” theme with lots of candles and paraphernalia. Have a buffet serving lots of neat foods and a theme punch. Check out a costumer to see what they have in their archives and to rent. Good luck!