Add some additional themes
Light and Dark mode are great but it would be nice to have some additional customization.
Jason R commented
This is really important.
There are many other email clients that have the ability to customize their style themes. I've been using emClient instead of Outlook for this reason alone.
I previously continued using Outlook 2010 for many years instead of Outlook 2013+ just because of the blue theme it had, as with the the changes from 2013 onwards to restrict styling to having too much white/light grey across the whole screen was too polarizing and made the messages list and reading pane blend in with everything else too much.
Its nice to see some grey come back at least so it gives more definition to the messages list and reading pane but please consider giving people the ability to customize it further.