[BUGFIX] Fix height of body in backend

The body-tag had 100% height in the backend. In views, where scrolling
is possible (e.g. login screen in a small window), this made the
background-color stop too soon.

Resolves: #53692
Releases: 6.2
Change-Id: I08df23c808b6b4d1ea7422a25a2431cf18d0ae70
Reviewed-on: https://review.typo3.org/25460
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters