I knew I wasn’t that out of it last night when I changed everything…
Turns out the reason the blog reverted to the default theme last night (and again this morning) is explained here. Basically, it had to do with me having two web servers both referring to the same DB, from back when I did the migration to the new server. I never turned off the old one, and some indexing service was apparently still using the old address to crawl the pages, and since the new theme was not on that site, WordPress would revert it back to the default in the database that they both shared.
Long story short… the theme should stay the way it is now. At least that’s what I hope the problem was…