I figured it out and hit reset in Advanced Options -- after deleting and uninstalling the new Outlook. I expect it to happen again, but at least I know how to recover Mail now. And I had to setup my account & signature from scratch.
Can't they at least make the new Outlook pretty? I want the functionality to use the Windows accent color as the theme color in Outlook, but no. Mail has it but it's too complicated for Outlook? Something that's built right into Microsoft Windows?? That daily color change makes my work life so much more palatable. I'm hoping I can hold onto it at least until 2024.