CKEditor config buttons not working in forced-colors mode

Problem/Motivation

CKEditor's "available buttons" are managed through admin/config/content/formats/manage/content_format the icons in the "available buttons" and "active toolbar" component are completely invisible.

Steps to reproduce

• Go here in your D11 or Drupal CMS site: /admin/config/content/formats/manage/content_format
• Enable forced colors in Edge or Chrome https://developer.chrome.com/docs/devtools/rendering/emulate-css/

Proposed resolution

Probably all of these buttons should be done with SVG files rather than background images.

Something like:
https://www.drupal.org/files/issues/2025-07-04/strikethrough-svgrepo-com... →

Or just use UTF characters like:

<span class="ckeditor5-toolbar-button ckeditor5-toolbar-button-bold">
  <span class="visually-hidden">Bold</span>
  <span class="forced-colors-label">B</span>
</span>

I've uploaded a bunch of possible UTF8 codes → other ones we could use here.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet