Hi MattMN,
Thank you for the posting in our forum community.
Regarding your description, I have checked this feature from my side but I could not see this feature
in my Outlook desktop client in Monthly Channel and after consulting with our team, I have got the information that, this sun/moon button option is available in
Office Insider version and for Office pro plus monthly channel version 1909,
it is in production for Outlook desktop client. Currently, there is no ETA because our related team is
working on it and it will see automatically in Outlook desktop client in Office pro plus monthly channel version.
But, regarding this dark mode and sun/moon button, it is available in
Outlook web app (OWA).
So, for workaround kindly login to Office 365 portal > open
Outlook web app > turn on "The New Look" toggle >click at the top of the right side Setting symbol in Outlook Web App > under turn on "Dark Mode" option and after that we can see sun/moon button in the Outlook web app.
Here is the screenshot of sun/moon button in Outlook web app:

I’m truly appreciating your cooperation in our forum community.
Regards,
Darpan