Thank you all for the testing. I've merged the changes into 2.x-dev and pushed a new 2.1 tag. @rudam could you please set 2.1 as the current recommended release?
Here's a patch that's a bit of a more comprehensive rewrite (removes all jQuery dependencies for pure Javascript) and adds D10 support for Composer. I don't have D10 up and running just yet, but it works in D9 and passes the code test in the official upgrade status module. As soon as this gets checked, I'd be happy to get it released.
I'm not sure how I feel about this, since this gets away from an on/off switch. Maybe as a feature enhancement, there could be a different template with a select box instead. As it stands, if the user never interacts with the toggle, it should always use the OS default.
This didn't quite work, but I will have a new version that adds Drupal 10 compatibility out shortly. Thanks for filing the issue.
I looked into this and don't see any problems. I'll integrate it into my Drupal 10 refactor.