For your decades party, celebrate a different decade every hour. Your background TV shows/music/games/food can change depending on the hour. For that reason, maybe it's not a good idea to decorate to the nines unless you have people helping you swap things out. Or, if you're planning on a full-group dinner at 8, you could at least make sure your dinnerware is all 70s themed. But don't worry about too much themed decor-balloons and streamers might be easier.