this is my first answer on this subreddit. Ill try to make my best. English is not my mother langue, some terms can be all over the place.

Territory organization in the Roman empire (before III-V crisis in Occidental R.E. and before Justinian 527-565) was made and controlled around the "city". The ancient city had the most important role in territory organization and power distribution.

Justinian territory organization changed that in a way and posterior conflicts in the East frontier of the Empire consolidated a new system:

firstly, the " eξαρχος" system. Cities under Justinian (and the whole Byzantine Imperium until its decadence) were still the most important part of the imperial organization. Byzantine imperium tried to develop a great set of functions that cities (specially the capital) would lead. There was still a controlled and voluntary effort from the imperial power to develop civical politics.

For that reason, "countryside" was left apart, and during Justinian that wasnt a problem, until he needed tributes and population for his armies in his expansive policy.

εξαρχος were created to improve imperial control in the provinces "far away" the capital of imperial power. both political and militar power was indeed in hands of the "exarch"?

Themas were an expansion of that idea: a political and organizative model designed to improve the imperial power under territories away from the most important cities. They were a " natural" answer to the actual need of fiscalization during expansion times.

They are also related to iconodule/iconoclasm problem. but thats kind of long and difficult to put it down without the actual vocabulary.

Source is Georg Ostrogorsky's book, mainly, and some other knowledge i got in my degree.