A default font is applied when creating your content.
Navigate to the Editor
Select the Settings (Gear) Icon
From here you can select the Base and Heading Font: The Normal Text will be applied for all fields that have content, lead/body text, lists, etc.; The Heading Text will be applied to all fields that have the Heading and Subheading Fields
Select the Font and tick Save. The font will be applied to the entire communication (content).
Tips!
You can add your own custom font
Set a specific font at the Account setting which will be automatically applied to all templates. Keep in mind that users still have the ability to override this at the Template/Content level.