Can I add my own css files to ckeditor?

TYPO3 Questions Backend (UI / Configuration / Permissions)
2

Hi Nils,
you can add custom CSS via the “contentCss” attribute in a custom ckeditor configuration yaml file which you have to create.

editor:
  config:
    contentsCss: ["EXT:yourExtension/yourPath/Css/ckeditor.css"]

How to create and configure your own configuration File is explained here:
https://docs.typo3.org/c/typo3/cms-rte-ckeditor/master/en-us/Configuration/QuickStart.html

If you dont have your very own config for the ckeditor, be sure to import the existing default config from the ckeditor to your config file.