The background of long text fields is white for basic and full HTML text formats in dark mode

Problem/Motivation

Noticed by accident, in dark mode the background in long text ckeditor based fields is white in dark mode on node edit forms for full and basic html text formats:

Update:

Turns out the problem was introduced with #3633459: Update CKEditor to 49.1.0. I went through the history looking for any issue related with ckeditor changes. the linked one was for updating ckeditor to version 49.1.0. I've discussed the problem with @kentr, and he was able to reproduce the issue after updating to the latest changes of main. the problem is not browser specific, is reproducible in safari and edge. he suggested to make sure it would be good to checkout specific commits:

git checkout -f acfffb4307a^ => the long text field is styled correctly
git checkout -f acfffb4307a => the long text field styling is broken in dark mode in default admin

Which is the proof that something broke with the update to 49.1.0

Steps to reproduce

Create a content type with a long text field and create a node of that content type

Proposed resolution

Ensure that the background for long text fields is not white for any text format and it meets the style of dark mode every time and in consequence that all text within that long text field has a color contrast of at least 4.5:1

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet