[TASK] Increase contrast for improved readability

The background color of the main body content is defined as a light gray.
To improve readability for content the background color is lit up.

Fixes: #47562
Releases: 6.2, 6.1
Change-Id: Ib7c9e3be4aba0069cda537071447bbaefdf39883
Reviewed-on: https://review.typo3.org/20300
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn