If your goal is to understand the music theory in "The Complete Idiot's Guide to Music Theory," then no: nothing has changed.
It depends on how you define "music theory." To most of the world, "music theory" is really just "the fundamentals of music": how to build this chord, how chords progress from one to the next, what a given phrase structure is, etc.
But if you mean academic music theory, there are two areas that I would say are the two biggest developments for the non-academic.
The first is called Sonata Theory, developed by James Hepokoski and Warren Darcy. It's a different look on "sonata form," and one that has been very popular ever since its introduction. It's hard to summarize in one sentence, but it's basically a goal-driven approach to sonata form that tries to relate a given piece to other sonata movements of the time. If you're interested in checking it out, this article would be a good starting point. If you're still curious, check out this subsequent article. And if you want more, check out their book.
The second is called Schema Theory. This is again hard to summarize, but it basically explains compositional practice (especially in the Galant) by looking at how these composers were taught. This article is helpful, but it's a bit outdated; your best bet is to check out the book.