Umm.. nothing? What do you mean in empty slots? If they are empty then there is no works, so you can't get a themeing bonus.
with the exception of Museums, all wonders/national wonders that have more than 1 slot for art work have some sort of themeing bonuses.
By default, all works provide +2 tourism and +2 culture (or +1 I don't know).
The Lourve has the highest possible themeing bonus with +4, and up to +8 with the Aestethic Finisher, and if you are France that jumps to +16 in their Capital.