[BUGFIX] Set background color for body and iframe

To avoid flickering in the TYPO3 backend, the body and iframe elements
are now styled to have a background color matching the selected color
scheme.

Resolves: #104917
Releases: main
Change-Id: Ib5c7ee67e29912f35aa97fb4ef3718b9cee32bee
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/86025


Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Nedbal <andy@pixelde.su>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>
Reviewed-by: Andreas Nedbal <andy@pixelde.su>
Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>