I've been on a real Byzantine rabbit hole the last day or so and I still can't wrap my head around what exactly what the theme system was. Was it is sorta of like having a state national guard in each region, whereas citizen soldiers were given land in exchange for service/training and a cut on their salary during campaigns? Was this advanced/better for its time and I'm wondering how it affected the quality of their military in comparison to others who didn't adopt such practices.
In order to understand the Thematic system fully, we must first understand the strategic need for it. The Thematic system was created as a result of the Arab conquests of much of the empires eastern territory and the prevalence of Arab campaigns into Anatolia. With the loss of so much territory and the clear military advantage of the Arabs, a system was needed where troops could be mobilized quickly and on a provincial level to more easily counter Arab raiders. The Theme system was also useful in that these soldiers were paid in land grants rather then from the state treasury, which helped to reduce the cost of maintaining these provincial armies and allowing the emperor to afford some full-time regiments, known as Tagmata. Of course there were flaws to this system, which became pronounced as the Abbasid Caliphate went into decline in the late 9th century. With the lowered Arab threat, the Byzantine state found it difficult to use the theme armies in an offensive capacity, as each provincial army only numbered around 10,000 men, and using the theme armies of multiple themes exposed these provinces to attack. Another issue with the themes was political, in that any Provincial Stratigos with a grudge against the emperor had an army on hand with which to wage war.
Addressing the quality of the soldiers this system produced, we need to look at the military record of the empire between 640 AD and 1050 AD, which is overall an impressive record. The armies of the Eastern Romans were able to defeat Arab armies, oftentimes at a disadvantage, though there were of course numerous failures and defeats, most notably when the Arabs were able to bring a large enough army and push through the smaller theme armies to sack the city of Amorium in 838. Its also worth noting that the wars against the Bulgars were bloody affairs, with Byzantine armies able to decisively beat the Bulgars but also to be routed themselves, such as at the Battle of Pliska. The armies that came after the dismantling of the Themes and the defeat at Manzikert in the 12th century Komnenian rulers were however also of excellent quality in there own right, though they were a standing army rather then hybrid system of the previous dynasties.
Sources: "Byzantine Wars" by John Haldon
"Tactica" By Emperor Leo VI
"Sowing the Dragons Teeth: Byzantine Warfare in the 9th-10th centuries" By Eric Mcgeer