If you're running iOS 12 or below, go to app Settings. Under preferences you'll see a Dark Appearance toggle.
If you're running iOS 13 or above, by default Outlook for iOS on version 4.5.0 or higher will respond to the iOS system Dark Mode setting. If you want to change this, go to app Settings > Appearance, and then select either the Light or Dark option.
To respect the iOS system Dark Mode setting, choose System Default. For more information, see the article Use Dark Mode on your iPhone, iPad or iPod Touch.