Yeah, it really is just asthetics. It's actually interesting how the game handles this. While the game's UI and background will always be based on whatever theme you've chosen, the pieces will show up as a mix of the 2 themes chosen by you and your opponent. So in one my current games I've chosen the Space theme while my opponent must have chosen the Prehistoric theme as all of the pieces show up randomly with either the Space or Prehistoric themes. I myself don't even own the Prehistoric theme but clearly am allowed to see it as that is what my opponent has chosen.