"Thematic" means, for us, using thematic (or topical?) linkages to tie history together. It is contrasted to a chronological approach, in which emphasis is given to dates and specific events, and to a regional approach, in which the course addresses the past one region at a time. 

Example: environmental change over time, or development of commercial systems over space and time. It could be political too, as with changing structures of empires over time.

Designing World History course: questions and answers