Okay. Breathe. :)
A nursery theme is really not that important. Just pick your furniture, pick a wall color, pick some cute bedding. If you don't have art, frame some of your favorite photos (or hang up some blank frames and put baby photos in them when the baby is born).
Plus, if you're going to find out the sex in three weeks, what's the big rush? If you just want to look, pick out a "girl" bedding set and a "boy" bedding set and put it in your registry. When you find out what it is, take the other one off.