Breaking: #106964 - Enable "Light/Dark Mode" context awareness for CKEditor RTE by default 

See forge#106964

Description 

With forge#105640, context awareness for the CKEditor Rich Text Editor (RTE) was introduced, allowing the editor interface to automatically adapt to the user’s system-wide light or dark mode preference.

This feature is now enabled by default. The TYPO3 Core stylesheet EXT:rte_ckeditor/Resources/Public/Css/contents.css has been updated to support light and dark mode variants automatically. Previously fixed white backgrounds now adapt dynamically based on the editor’s preferred color scheme.

Note that this change affects only the backend editor interface. The display of RTE content in the frontend remains unaffected.

Impact 

The previously fixed light mode user interface of the CKEditor RTE is now context-aware, displaying content in light or dark mode according to the editor’s system preference.

Affected installations 

TYPO3 installations that rely on a fixed light mode presentation of CKEditor RTE instances in the backend are affected.

Migration 

Installations with custom CKEditor modifications should review their contents.css file. If the TYPO3 Core default stylesheet was previously used, and a fixed light mode appearance is desired, this can be enforced in the RTE YAML configuration:

EXT:my_extension/Configuration/RTE/MyCKPreset.yaml 
editor.config.contentsCss:
  - "EXT:my_extension/Resources/Public/Css/CustomContents.css"
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