what's your budget for the party?
were you planning on serving food? Something a little more impressive than chips and dip?
theme parties I've always thought were a little silly... I don't have the patience or creativity to put together a costume... but black and white parties are generally pretty easy..
Back when i was in college, one of our frats used to have a black and white party every year... to gain admission you had to wear black and white (and didn't have to be dressy) Cocktails served were black and white russians... kinda fun...
If you want to go with a theme - I'd go with the year that she was born... 1985? (good lord I'm old...) play music from that era... have people come as their favorite movie character from that year...
http://movies.yahoo.com/browse/year/1985
lists the movies from '85 - back to the future was from that year... perhaps that could be a theme...
The year of your girlfriend's birth was the year the hit movie back to the future was released... so therefore -- pick where in the past you're going back to (even if you pick somewhere in your girlfriend's past...)