[BUGFIX] FormEngine & DynTab styling and contrast

Fixes DynTabs where background and tab body content
lack on necessary contrast - especially FormEngine.

Resolves: #62002
Releases: master
Change-Id: Ib762306c0d825a77cdbb4255f51de1c30d7d6e76
Reviewed-on: http://review.typo3.org/34822
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>