First of all: my apologies if this is the wrong forum to be asking this question.

I've been trying to find comprehensive information on creating a KWin theme for a while now. The closest I've come was this, but it makes a lot of leaps that I can't follow. Is there anything comprehensive? I'd be especially happy if there was example code and documentation telling me what each code file does.

KWin Decoration API will change for KWin 5.x, so you might want to delay your efforts.
The most simple way is to pick an existing decoration and "fork" it until it's like what you want.