City of light

I do not really understand what city of light really does. Does it double the tourism in Paris or what is it?

It doubles the theming bonus. That's when you have a certain combination of types of Great works in buildings with two or more great work slots. For example, two great works with artifacts from the same civilization and era in a museum, for a +4 tourism bonus, +2 being the normal bonus

Thank you so much, now I also finally see the point in trading great works and why it can be better to pick a certain artifact with an archeologist!

Or the Lourve, which has a possible of +16 themeing bonus

base +4 from 2 Different Paintings from different eras and civilizations paired up with 2 different artefacts from different era and civilizations.

Which is automatically doubled into +8 with their Unique Ability, and then further upgraded into +16 with the Aestethics finisher.