I get this message in advanced options and I really don't understand what I'm supposed to do:
It seems your theme doesn't support customised styles for the editor. You can create a CSS file named editor-style.css and upload it to your theme's directory. After that, enable this setting.
I created this new CSS file, checked the 3 offered options but nothing happens, the file remains empty.
What are those buttons for: Remove, Import, Export settings - what settings, import from where, export to where...?