CKEditor

Ckeditor5 Stylesheet

https://www.drupal.org/node/3259165

mantra_starter.info.yml

This stylesheet is loaded with ckeditor.

ckeditor5-stylesheets:
  - css/drupal/ckeditor5.stylesheet.css

elements.css

Base styles set and use variables.

blockquote {
  --blockquote-border: .25rem solid var(--color-dark);
  --blockquote-font-style: italic;
  --blockquote-padding: 0 1rem;
  --blockquote-margin: 1rem 0;
  padding: var(--blockquote-padding);
  margin: var(--blockquote-margin);
  border-left: var(--blockquote-border);
  font-style: var(--blockquote-font-style);
}

ckeditor5.stylesheet.css

Prefix items with .ck.ck-content to override.

.ck.ck-content blockquote {
  padding: var(--blockquote-padding);
  margin: var(--blockquote-margin);
  border-left: var(--blockquote-border);
  font-style: var(--blockquote-font-style);
}

Build ckeditor from source
https://www.drupal.org/docs/core-modules-and-themes/core-modules/ckeditor-5-module/ckeditor-5-development

Default CKEditor styles
https://ckeditor.com/docs/ckeditor5/latest/installation/advanced/content-styles.html#the-full-list-of-content-styles

CKEditor theme customization
https://ckeditor.com/docs/ckeditor5/latest/framework/guides/deep-dive/ui/theme-customization.html

Stackoverflow thread
https://drupal.stackexchange.com/questions/319756/how-can-i-modify-ckeditor-5s-css-variables