Migrate CKEditor styles mechanism (BE)

Customers use own css and font files which they load into CKEditor via configuration attribute contentsCss. Loaded css files can reference font files (and imgs).
https://xmdocumentation.bloomreach.com/library/concepts/document-types/html-fields/customize-ckeditor-css.html

TinyMCE needs to be investigated with regards to support of custom styles. Then, the css files (used currently in CKEditor's mechanism) and its registration in the editor config need to be migrated to what is equivalent for TinyMCE.

Custom CSS files most probably contain generic rules but they can also contain workarounds and fixes for issues specific to CKEditor; this should be investigated and perhaps it suffices to merely give a recommendation (to clean up) to customers.