Released v0.1.0-beta.11.1 today to fix the issue with saving css (and many other settings).
Run this command in case you are already on beta 11. If you are upgrading to beta 11, this version will automatically be installed.
composer update flarum/core
Confirm you are running the correct version in your admin dashboard or through
php flarum info.
Thanks everyone who reported this, we've added this to our Q&A checklist for future confirmation.