Well I have to say, I may have been mistaken when I said a previous version had the reading pane in a different color.
I booted up my old machine that I thought had the reading pane in a gray color, but it is currently white as well. So either I am misremembering, or it turned white when it updated, at some point.
My current machine is running this version
Microsoft® Outlook® for Microsoft 365 MSO (Version 2407 Build 16.0.17830.20138) 64-bit
My old machine is running this version
Microsoft® Outlook® for Microsoft 365 MSO (Version 2405 Build 16.0.17628.20006) 64-bit
The reading pane is bright white in both versions.
EDIT: I will check on the machines of some of my employees on Monday