Follow up to #1890502: WYSIWYG: Add CKEditor module to core.
The PHPDoc of hook_ckeditor_css_alter() currently says:
* Front-end themes (and base themes) can easily specify CSS files to be used in
* iframe instances of CKEditor through an entry in their .info file:This, however, doesn't make clear what situation this is needed for. It's for when a front-end theme wants to add to the CKEditor's iframe CSS even when the editor appears in an admin theme.
In #1890502-61: WYSIWYG: Add CKEditor module to core, quicksketch says:
Often times the front-end theme uses a significantly different font and colors (say a dark theme instead of a light one). So the front-end theme may want to provide CSS that is used in the iframe of the editor, so that the appearance lines up more closely to the front-end.
We should refine that some and include that in the docs, and improving the corresponding examples for the .info and _alter() approach.