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

See forge#106964

Description

With forge#105640 context awareness for CKEditor RTE was introduced, but not enabled by default to not break existing customizations/enhancements of the editor interface.

The TYPO3 Core default stylesheet (EXT:rte_ckeditor/Resources/Public/Css/contents.css) has now been adapted to enable the context awareness by default. Prior white backgrounds are now shown as either dark or light backgrounds, depending on the editor's preference.

Please note that the display of RTE contents in the frontend is unaffected.

Impact

Prior fixed "light mode" UI display is now made context aware, showing CKEditor RTE instance contents in the editor's preferred dark/light mode variant.

Affected installations

TYPO3 installations relying on "light mode" UI presentation of the CKEditor RTE instances in the backend.

Migration

Installations with custom CKEditor changes should review their contents.css file. If previously the TYPO3 Core default sheet was set, and a fixed "light mode" is preferred, this can be achieved by adjusting the RTE YAML configuration:

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