[BUGFIX] FormEngine background color and padding

Adapts backend color for FormEngine footer cell to white.
Also removes an defective margin above FormEngine tabs.

Change-Id: I5500a2c51fcffe4ff00d197d6b6480cb37bad8a4
Fixes: #47894
Releases: 6.2, 6.1
Reviewed-on: https://review.typo3.org/20474
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn