[BUGFIX] Styling fix for Install Tool Lock Screen in Backend Mode

Resolves: #55464
Releases: 6.2
Change-Id: I1891763db8a8edc5ddb8077a1cc32c8184a7e0c1
Reviewed-on: https://review.typo3.org/27156
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny