A theming bonus is a Culture and Tourism bonus received by filling Museums with Great Works of Art or Artifacts that satisfy certain criteria. When activated, the bonus doubles the yields of all items in the Museum.
When playing as Kongo, the Food, Production, and Gold bonuses also get doubled if you manage to theme an Art Museum with Sculptures or an Archaeological Museum with Artifacts. Theming also doubles the Science bonus to Great Works granted by the Anshan city-state.
To receive the theming bonus from an Archaeological Museum, its slots must be filled with Artifacts from the same era and different civilizations. Each city-state and Free City counts as a distinct civilization for this purpose, as do the barbarians.
To receive the theming bonus from an Art Museum, its slots must be filled with Great Works of Art of the same type (i.e. Sculptures, Portraits, Landscapes, or Religious) made by different Great Artists. This means that a minimum of three Great Artists are needed to activate each Art Museum's theming bonus.
In Gathering Storm, Kristina's leader ability automatically themes all her buildings with at least three Great Work slots and all her wonders with at least two Great Work slots once all their slots have been filled.