I'm glad you figured out the Set as Default button, that's great for minor changes.
Outlook uses the NormalEmail.dotm file in your Office Templates folder to store the formatting defaults for your emails. You can edit that template in Word to set your default fonts, colors and styles for new emails. The NormalEmail.dotm file is stored in C:\Users[YourActualUserName]\AppData\Roaming\Microsoft\Templates. Close Outlook, open the template in Word using File>Open, edit and save it. Then open Outlook and create a new email to test.