What does "if themed" mean ?

[Beniito]

this may sound like a dumb question, but what does it mean when a building gives an extra bonus if it is themed ?how do I theme a building ? for example :
Great library : 2 Great Works of Writing slots (+3 if themed)

 

[Grabbl]

You get a theming bonus if you fill the Great Work slots with appropriate works. You can see the condition on the Tourism Screen (tab for great Works) by hovering over the slots. Usually it is a condition like "All Great Works need to be from different civilizations than the city owner" or "All Great Works need to be from the same civilization and era". Theming gives you additional culture, tourism and the above theming bonus.

 

[Beniito]

I see thank you

 

[grmagne]

By the way, if you need works from other cultures to get your theme, there is a tab that allows you to trade great works with other civilizations.