Using New Outlook on Windows for personal email, calendar, and contact management
Try going to the gear icon top right where the minimize/maximize/close buttons are
"Accounts" should already be highlighted in the window that pops up.
Select the gear icon (General) right under "Accounts"
You'll see another window open up with "Language and Time" at the top.
Directly under that is "Appearance". Click on that and make your changes.
Hope that helps