I find that is mostly a matter of opinion... no?
(We've been struggling with themes for a LONG time in the playkingdoms remake
)
Yes, apart from Geobra's own catalogues, which put sets together in themes, it is a matter of personal opinion. I sometimes use PlaymoDB's themes, but some are less useful ('Knights' covers a wide variety of sets, 'Traffic' covers some but not all vehicles, 'Rescue' covers both 'Fire and Rescue' and Hospitals, 'Free Time' and 'Leisure' seem to cover similar sets and so on. I guess some of the themes reflect different phases in PM development.