If you want to change the background color on an email, Outlook’s option to do that has changed name and location. At least Windows users can change background color, Outlook for Mac users lost this option at few years ago!
A colored background can be nice in an email but care is necessary when replying to an email with a color background. The ability to change the email background is more than a design choice, it can be necessary to make it readable.
Back Outlook 2003 and earlier email background was obvious under Format | Background. In Outlook 2007 and later it changed to Options | Themes | Page Color which is the same thing with a different and misleading name.
Why is an email background option called Page Color? Outlook’s email editor is really a scaled-down version of Word. Microsoft appears to have just copied the background color feature from Word without changing the label to something more appropriate.
Missing from Outlook for Mac
Ever since Outlook 2016 for Mac there’s been no background color option in Outlook for Mac. It’s totally missing from Mac Outlook 365, 2019 and 2016.
That’s a real problem if you’re trying to remove or change a background when replying.
If you’d like it restored, add a vote to User Voice though it’s doubtful how affective more votes will be. After all, Microsoft has been ignoring this simple and reasonable customer request for at least four years and many revisions of Outlook.