[BUGFIX] Restore gray background in EXT:dashboard

The patch for issue #97144 explicitly set a white background
for all modules to improve scroll performance. It introduced
a regression, changing EXT:dashboard's background from light
gray to white as well.

This patch resolves the regression by applying the gray
background to the new element.

Resolves: #97713
Related: #97144
Releases: 10.4
Change-Id: I411b817a7d020a144c43d4382574c7a5d21e554e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74921


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>