As discussed in Chapter 2, The Content Authoring Experience, Drupal ships with the WYSIWYG support and CKEditor as the default editor. The CKEditor module will inspect the active theme and its base theme, if provided, and load any style sheets defined in the ckeditor_stylesheets key as an array of values.

For example, the following code can be found in bartik.info.yml:

ckeditor_stylesheets: 
  - css/base/elements.css 
  - css/components/captions.css 
  - css/components/table.css 

This allows themes to provide style sheets that will style elements within the CKEditor module to enhance the what you see is what you get experience of the editor.