Additional CSS not working after WordPress update
Quite often, once you update your WordPress version, you start to notice some strange things happening. The reason for this could be compatibility issue between the newest WordPress version and an older version of a plugin or a theme on your site.
However, I noticed that the issue with Additional CSS not saving is related to a specific option in your WordPress backend.
Here’s what to do in order to bring back the Save functionality for Additional CSS:
1. Go to WP Dashboard -> Users -> Your Profile.
2. For Syntax Highlighting check Disable syntax highlighting when editing code.
3. Go back to Appearance -> Customize -> Additional CSS. Now, once you make a change there, you will be able to save those changes:
Hope this helps. As always, let me know your opinion in the comments below. Cheers!