It's reasonably well-structured. So every time I beat my head against a particular wall long enough, I finally have an a-ha moment that clears up lots of things.
Themes inherit from the base system. Child themes inherit from parent themes. CSS cascades down through ridiculous numbers of layers and still works. (Assuming you found the correct selector to begin with.) The particular theme I'm using is almost 100% structure with very little design.
It all just seems to work. Even caching pre-compiled pages.
Oh, and I'm pretty sure I found my hosting solution. Now, to figure out the easiest way to automate the cleanup after I import the current blog ...