There are many ways to customize the appearance of Outlook. Perhaps the change in appearance you're noticing is due to one or more setting changes.

For some weird reason, Microsoft thinks it's wise to distribute these kinds of settings willy-nilly in different areas of the application. Here are a few places to check:

In the main Outlook interface: Check the "View" ribbon.

Or go to "File" > "Options" > "Mail" > "Editor Options..." and/or "Stationery and Fonts.."

If you open a new email message, check the "Options" ribbon for ways to tweak the appearance.