Using Classic Outlook on Windows for personal email, calendar, and contact management
I've got the same issue, I can change to light background for individual messages by clicking the sun symbol
And the background goes white
But I have to do this for EVERY email, even reopening the same one it'll default back to a dark background.
It's feasible as the white background in dark mode works, so shouldn't be too hard to make a setting of Light Theme/Dark Theme AND Light Background/Dark Background as default settings.