CKEditor in Civicrm - Is it possible to have a custom css file?
If you use the great CKEDITOR editor to configure what you can there, then save, you can then edit the file in (typically, for a D7 implementation): sites/default/files/civicrm/persist/crm-ckeditor-default.js
e.g.
CKEDITOR.editorConfig = function( config ) {                                 
  config.allowedContent = true;                                              

  // Added these lines: ↓
  if (typeof config.contentsCss === 'string') {                              
    config.contentsCss = [config.contentsCss];                               
  }                                                                          
  config.contentsCss.push(&quot;/sites/all/themes/your-theme/css/ckeditor.css&quot;);  
  console.log(&quot;I hacked CKEDITOR's CSS :-)&quot;, config.contentsCss); 
  // ↑ you don't need that console.log, but it's handy to prove it's working.
};                                                                           

However, do note as it says at the top of that file, Note: This file will be overwritten if settings are modified