This is probably a very under looked aspect concerning games and their creation, yet it's something that we look at the whole time we're playing, so it's a pretty important aspect. What type of "feeling" would you want the GUI (buttons, frames, etc) to be? Would you want it to be dark and forbidding? Classical in style? Would you want a color scheme to make you feel like your in that time period? If so, what colors would best represent that?