[BUGFIX] Dim not selected crop area again

While cleanup css with #79998 the background was accidentally removed.
Add it again to make the selected crop area more prominent again.

Resolves: #81503
Releases: master, 8.7
Change-Id: I197d6c4bb9b7be7d2b9a443ce1b60ab3b4d82bcd
Reviewed-on: https://review.typo3.org/53156


Reviewed-by: Jasmina Ließmann <code@frauliessmann.de>
Tested-by: Jasmina Ließmann <code@frauliessmann.de>
Reviewed-by: Robert Vock <robert.vock@4wdmedia.de>
Tested-by: Robert Vock <robert.vock@4wdmedia.de>
Reviewed-by: Helmut Hummel <typo3@helhum.io>
Tested-by: Helmut Hummel <typo3@helhum.io>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>