Apologies for the delay here, we have reproduced the issue in one specific scenario and merged a fix. If you are a hosted customer, the fix will be in your instance over the next week or so.
The specific scenario for the repro is using the default Outlook Web app running on an OS set to dark mode.
That combination caused some dark mode styles to apply to elements in the email template. That is because Outlook Web will respect @media (prefers-color-scheme:dark) (since OS is in dark mode) even though Outlook Web itself is not in dark mode. This is pretty much a bug with Outlook Web.
Nonetheless, we’ve implemented a minimal fix. Here’s the relevant commit in core: