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But you are correct in that one of Tolkien's great themes is the corruption of power.

Well, Aragorn had power and used it without being corrupt. Ditto Theoden. Denethor had a problem, but he lacked humility and attempted to challenge Sauron with one of Sauron's favorite tools. I don't think Tolkein's theme was power corrupting but the necessary humility for handling power. And one element of humility is that magic isn't for human beings. At least, that's what I got out of it without reading the appendices or The Silmirilion.

Shalom.

Well, Aragorn was in his 70's and had not pushed for status or honor, so he had learned restraint and humility. Theoden had become weak and had started listening to Grima. It was only with Gandalf's interference that Theoden showed the strength and power he did in his youth. Denethor was wise and restrained, but Sauron held the master Palantir and was able to slowly corrupt him (as he did Saruman).

Aragorn, revealing himself as the rightful heir, took a large risk at Helm's Deep by using the Palantir, but it was to force Sauron's hand to strike before he was completely ready. His first action as a proclaimed heir was to go to the Stone of Erech and claim the fealty that the dead owed Elendil. After the defeat of the Corsairs, he again hid himself until his coronation. Aragorn was one of the wisest characters in the story, and as such corruption was held in check. He even told Boromir that he wouldn't hold the ring under any circumstance because he feared the corrupting nature of it.