If you are asking about email you send with Comcast's Open-Xchange "Appsuite" webmail ("...connect.xfinity.com/appsuite/..."), see https://forums.xfinity.com/t5/Email-Web-Browsing/Email-Default-Settings/m-p/3142997#M202197.
How can I make changes to the default font and font size in comcast email?
In Xfinity.Connect webmail, click on the gear icon [next to the question mark], then on Settings.
In the right pane scroll down to Default font style
Click on the three choices to make your selections.
If this doesn't "take" clear your cookies for xfinity.connect, close your browser, start it in a new session, and make your changes again.