Thank you for reaching out to the Constant Contact Community. It's my pleasure to assist you today.
At this time, there isn't a way to edit the colors and fonts within the ListBuilder app. This is feedback we are tracking for our team and I have submitted your request to them. Please feel free to reach out should you have other questions. Thank you for choosing Constant Contact. Have a good day!
I am surprised that something as basic as changing the fonts and colours was not included in the app. That should be something that is part of personalization.