For those of us outside NA - please explain the mechanics of the theming bonus. Thanks in advance.
For wonders/buildings that can have more than one Great Work placed, they generally have "theming bonuses" that add bonus tourism when their conditions are met.
For example, the Great Library has 2 spaces for Great Writings. You can place any 2 Great Writings in those spaces, and each will give you +2 culture and tourism.
However, if you place 2 Great Writings that are from different ages and sources, you get a "theming bonus" (I think for the Great Library it's another +2) additional tourism (don't remember if it's also additional culture).
So, if you have a GL with 2 Great Writings that are both from your culture (you created them), or from the same era (the era they were created, e.g. Medieval or Renaissance), or both, you won't get the theming bonus, so the GL will be producing +4 tourism (and culture). If you go ahead and trade one of those Great Writings with another empire, and acquire in trade one that is from that other empire, and from a different era, as the one you still have in the GL, your GL will now be producing +6 tourism.
Theming bonus requirements are different for different buildings. Some require the works to be from the same era and empire, some from a different era and empire, some require different types of works, etc. to achieve that buildings bonus.