[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/20696
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
[TASK] Must use "self::" for local static member reference

Resolves: #47926
Releases: 6.2, 6.1
Change-Id: I89aaf7e6fe75d66e3b3769c6a1b6ecd04e8b6df5
Reviewed-on: https://review.typo3.org/20697
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
[TASK] Cleanup: Must use "self::" for local static member reference

Resolves: #47925
Releases: 6.2,6.1
Change-Id: I8b23f014fb215a262801480bd4bda3bc44f8c11c
Reviewed-on: https://review.typo3.org/20698
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters