Think of a theme as an area ruled by a Military Governor/General who had full control of all forces (standing army and militia levy) who had to report to the Emperor. This Governor/General was also in charge of administering the entire theme, indeed, the theme system came about because centralized power from Constantinople could not respond quickly to cases of bandit attack or raids on the frontiers. By delegating power to a subordinate on the ground, the borders could be defending.

A legion was just a standing army. No land was under their control, all land was under control of the Roman Empire.

The problem is that the word Theme seemed to be used as a catchall, so it is very hard to determine what was a Theme's role and what was the civilian administration role.

I'm not up to date on the origins of the theme system as I believe it's going through a bit of a revision, but it basically emerged around the time of the Arab conquests (7-8th centuries) once the Empire had been reduced to Anatolia and some scattered Balkan and Italian territory. At that point, there would be regular Arab raids into Byzantine territory so there was a need to defend against those vs. maintaining mobile field armies like in late antiquity.

So these armies were settled in Anatolia and each had a district (i.e. theme) to defend. Soldiers were settled on enough land to support them and their families and in exchange they would have to perform military service. Originally the service was inherited through families, but then the obligation became attached to owning the property itself.

Themata (pl. of theme) were typically headed by a strategos ("general", but think more like a governer-general), who was appointed by the Emperor and received a yearly salary. Unlike in the west when ostensibly miltary titles like count and duke became hereditary, strategoi did not. The first themata were quite big and heading one during the early years gave would-be usurpers a leg-up on attacking the central government. As the empire recovered and expanded during its middle period, new themata were created and older ones broken up to help prevent this. The Emperor also retained some guards regiments (tagma, pl. tagmata) around Constantinople as a permanant standing army. These groups were small (a couple of thousand), but elite. The famous Varangians were a later tagma.

During a war, the Emperor would call the strategoi to mobilize and either he or his commander-in-chief (the domestikos ton scholon, commonly anglicized as "Domestic of the Schools") would command them.

Towards the end of the 10th century, two trends led to the disappearance of the typical thema: wealthy provincial magnates started to buy up the soldiers' properties to add to their vast estates without providing the necessary military forces. Emperors attempted to combat this with legislation aimed at restoring lands back to soldiers with mixed success. And the frontier moved from Cicilia and Anatolia east into Syria and Armenia, making the interior themata unnecessary from a defensive perspective.

The Byzantine Empire experienced a prolonged political crisis during the 11th century which resulted in the loss of Anatolia, which more or less permanently extinguished the themata of any consequence left.