change background color to grey

[martinbuergi]

Please always provide the GitHub issue(s) your PR is for, as well as test URLs where your change can be observed (before and after):

Fix #

Test URLs:

• Before: https://main--wknd--hlxsites.hlx.page/
• After: https://optimize-carousel-text-color--wknd--hlxsites.hlx.page/

[aem-code-sync]

Hello, I'm the AEM Code Sync Bot and I will run some test suites that validate the page speed.
In case there are problems, just click the checkbox below to rerun the respective action.

• Re-run PSI Checks

[aem-code-sync]

Page	Scores	Audits	Google
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