Outlook for Office 365 (the click to run version) now supports the Office Black theme. Preferring to use dark Outlook is very much a personal choice. I’m not convinced that I like it, even if you can force the message reading pane to be in light mode.
I've never been a dark mode type of guy, until lately. They are actually looking fairly nicely designed and I've been finding most of my stuff using it. Glad to finally see the preview pane getting the dark background like Windows 10 mail and Outlook web have. Hasn't quite hit monthly yet.