I'm not sure I saw any evidence of Sir Terry having a central theme in
mind as he wrote the series, apart from the obvious fact that they were all written by one man with one particular worldview and set of beliefs about reality. (He certainly blurred the line between ruthless realism and whimsical fantasy in a truly genius way, as far as I'm concerned.) But to me it seems more as if he had
several recurring ideas and issues and even genres that kept cropping up throughout the series--ones that could each make up a thesis in themselves, probably! And like
@The Big Peat said, ones that evolved with the author over time. The Discworld series was huge.
Sir Terry himself once referred to the Discworld as rather like a whiteboard for him, for the stories he felt like writing at any given time. It was in a recorded interview from sometime in the 2000s. And I guess if I were writing a thesis about the ideas behind Discworld, that's probably what I would turn to first--all the recorded interviews Sir Terry gave throughout his life, talking about Discworld and his own life's influences on it, and the reasons behind some of his creative decisions. There are tons of them on Youtube; I spent a couple of hours downloading a couple dozen of them a month or so ago, and I
still haven't watched through them all. I don't think I even saw the majority of them.
Just a few ideas. At any rate, good luck on your thesis! Any chance of it ever showing up where we on Chrons can find it, once you're done?