You can extend the design of your newsletter emails by adding HTML content. If you are not familiar with coding in HTML you can use any free third party software like BEE Pro to create the content and then just copy and paste the HTML created into your newsletter.
The article on Sending emails to your database describes in detail how to send a newsletter to your community. This below article explains how to add HTML content to the newsletter.
The below snapshot highlights the content area for your newsletter.
Click on the last icon on the editor to switch to HTML mode.
Now you can paste any HTML content you exported from any tool or write your own content.