I'm having this same issue. I use Outlook for work and school on a Windows 10 laptop and a 2013 iMac, both of which are fully updated. The sun & moon icons show up on my iMac, but not my Win10 laptop. Being that I do a lot of work at night, this issue is
incredibly frustrating.
How is it that the feature works on an Apple OS using the same Microsoft account, but not on a Windows 10 laptop? Oh yeah, and dark mode works perfectly fine with the native Win10 mail app.
What is the point of my work AND school organization paying for the premium apps if the premium apps can't keep up with this incrediblly simple feature from the free native app?
Hello
Can I refer you to Gabriel Batton's post on 15 November last year which solved this for me?
Gabriel: "From what I can tell the Sun/Moon feature is enabled 100% for Monthly Channel. If you click File, Office Account, under About Outlook what Build and Channel do you have?
If you are on Semi Annual Channel there are steps documented here to switch to Monthly Channel, https://docs.microsoft.com/en-us/office365/troubleshoot/administration/switch-channel-for-office-365.
The earliest the feature would get to Semi Annual is likely July next year per channel updates page, https://docs.microsoft.com/en-us/DeployOffice/overview-of-update-channels-for-office-365-proplus.
"
I hope that helps.