Yes! From the Review and Edit Message screen, scroll to the Message Body section, and locate the "code view" button. This will allow you to add in your own custom HTML, as well as see the HTML that is currently a part of the message.
(If you are not seeing this button, we do recommend viewing the page in a wider screen.)
Please note that when using the HTML editor in the Communications Center, certain tags could be stripped due to interactions with our WYSIWYG. You might need to adjust tags to achieve your desired results. At this time, our support team does not offer code level support.
We recommend sending yourself a test message by clicking "Save and send Test" ensure the desired result is achieved before setting messages to active.
Additionally, when adding your own HTML, please keep in mind Amazon's Allowed HTML and CSS tags.