Breaking: #61471 - EXT:t3skin CSS files moved to less

See Issue #61471

Description

All CSS files in EXT:t3skin are moved to less files and handled by less CSS pre processor and merged to a single CSS file.

Impact

Single CSS files can not be included anymore. This may result in broken layouts.

Affected installations

A TYPO3 instance is affected if an extension loads single CSS files from EXT:t3skin. Backend modules of extensions usually get CSS core stuff loaded by default, which will not be a problem. An extension is only affected if single CSS files are explicitly referenced. This should be a rare case.

Migration

Most simple solution is to copy over the “old” CSS file from an older instance. Directly including those files from t3skin is discouraged. A better solution is to refactor the extension to use the full t3skin CSS file and to overlay it with required changes in an own file.