[BUGFIX] BE login shows unaesthetic scrollbars

CSS adjustments for login screen in t3skin

Fixes: #43330
Releases: 6.1, 6.0

Change-Id: Ifeb141ff9963f54e35c26ca41bc73bc12d79b7c2
Reviewed-on: https://review.typo3.org/17067
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Felix Kopp
Tested-by: Felix Kopp
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel