Hi @i_mike,
I’ve just checked my own installation, with WordPress 5.6. The css files in the frontend and in TinyMCE seem to work fine, also adding a new custom style. But when I try to remove a custom style I get “Uncaught TypeError: document.getElementById(…) is null” in the browser console. Is this your problem?
I’ll try and have a look at it but no promises – so much to do…
Again: If anyone wants to take over this plugin, please let me know!
Tim