You can change your Outlook theme in your account settings while logged in through a browser or in the Mac and Windows apps.

Did you open Outlook today and think, "Why does everything look different?" Maybe the message list looks squashed, the reading pane is in the wrong place, or the whole layout feels unfamiliar. Happily ...

To change the default browser in Outlook 365, go to File > Options > Advanced, then scroll to “Link Handling” and set “Open hyperlinks from Outlook in” to your preferred browser, like Chrome.