What to wear to winter casual tail party at professor's home?

Hi, I need your help. I'm a girl, it is a party after a PhD student's Doctoral thesis defence (in physics) held at my professor's home in UWS Manhattan. There will be some other professors and senior students. It is said to be a casual, fun one. I have very short black hair ( a little bit like Victoria Beckham's )and I'm petite 5'3" 80Lb. I want a modern, smart, yet feminine, and casual look. Can you give me any suggetions and where to buy. Thank you so much!