The Rich(Floating) interstitial in-app message template can be customized using HTML and CSS. More details about this can be found here:
Customize Rich(Floating) interstitial in-app message template
We recommend creating and customizing it in the Messaging page as it allows inspection of the HTML elements in the message preview in Web Tools.
After you finish the design, you can copy the code and paste it in any of the text fields of an in-app created in the Campaigns page.
The following list contains the CSS selectors for the HTML elements you can style, indented as per the current structure:
#view (message view)
.image (background image)