That's a great question. Creating your email to match what you would like is definitely important. Because of this, I would like to apologize being able to edit the margins in your email is not currently an available feature. A workaround we can suggest is matching your Outer Background color to match your Inner Background color. Making this change will help the margins to not be as visible. We have also submitted your feedback on a feature like this in the meantime.
Frankie P. Community & Social Media Support
Did I answer your question? If so, please mark my post as an "Accepted Solution" by clicking the Accept as Solution button in the bottom right hand corner of this post.