I needed to fix a library-level theme item. And since it's been a while since I last made a change like that (this was all before the original switch to the production Drupal instance), I couldn't remember if it involved uninstalling/reinstalling the theme...or simply clearing the cache.
So I chose the heavy-handed option. What could possibly go wrong, right?
I'm certainly glad I don't have a lot of super-customized stuff (other than some titles and custom block layout stuff to place), because I had to do it all over again.
All of this to discover what I'd changed didn't take effect. Rinse. Repeat...except this time I just cleared the cache.
This time--it worked!
So, note to future self. Or others out there. Theme changes don't necessarily require a wipe and reload. Try the cache first!
And export a copy of the settings before you do anything!