Outlook appearance is too bright in regular mode and too dark in dark mode. Additional viewing backgrounds in preferences.
Regular mode viewing is too bright and too washed out confusing the reading pane with the sidebar and the titles panes. The reading pane should stand out without being too bright and the side bars and title panes to be more indicative of where the focus is. Similarly in dark mode, too dark and the different panes become a scrambled mess visually.
Joseph Hanna commented
Dark mode on Office 365/outlook.com is perfect. Dark mode on Outlook for Mac is too high contrast (background too dark and text too bright)
I'd agree - the Dark Mode conversation list and reading pane headers are very dark, almost black, with white text, which is hard to read. Proper Dark Mode is more of a subtle dark grey, not black and white.
Would it be possible to tweak the colour contrasts you are using so it better matches Dark Mode in other Mac apps.
Take a look at Apple Mail for an example of how it can be done well.