LiveJournal just mailed me a batch of comment notifications from two years and one month ago. Oooookaaaaay.......
In other news, freetrav mentioned having seen the theme-resetting behavior before, and said that in his case it was caused by a corrupted LiveJournal cookie, but he couldn't figure out which one, so he deleted ALL livejournal.com cookies and it fixed the problem. So, I did the same thing a few days ago. And HOLY CRAP there were a lot of cookies. I think there was a cookie from every journal I've looked at or seen a post from in the last six months. I wish Firefox cookie control was granular enough to let me say "Save my LiveJournal login cookie and cookies related to my account itself, and expire all other LiveJournal cookies at end of session." But it doesn't appear I can actually do that.
So, anyway, we'll see whether it works or not.
Note to LiveJournal: Maybe LiveJournal might behave more reliably if it didn't rely on quite so many damned cookies.....? I deleted almost a hundred of the cursed things.