Accepted answer

Hi, there is a change record with a section called "Changes for Themes that Provide CKEditor-specific CSS", see https://docs.backdropcms.org/change-records/ckeditor5-upgrade. Hope this helps!

Comments

I just found it in the parent theme (mine is a sub theme of basis). I'll see if this still applies to cke5. If not, then I still have the question about v5.

Nope. Basis has this in it's .info file:

ckeditor_stylesheets[] = css/base.css

Adding that to the sub theme's .info file can not style the editor.

Yes, the subtheme, and possibly basis on a fresh install of bd-1.27.0, will need something similar like:

`ckeditor5_stylesheets[] = css/base.css`

you might also want to keep, for cke4,

`ckeditor_stylesheets[] = css/base.css`

if switching back and forth from cke4 and cke5.

Thanks so much @Olafski + @izmeez !

I only glanced at the change record + didn't get past upgrade process :-(