How can I center the in-app message display with margins relative to the screen?

Regarding the in-app design, I would like it to be displayed in the center of the screen, with a margin in relation to the screen size. 

Here is an example image of an in-app made using another tool we have, One Signal:

Is it possible?

This documentation (https://docs.oracle.com/en/cloud/saas/marketing/responsys-user/Inapp_Design.htm) explains about the message style, which can be Full Screen, Alert, Interstitial, Banner-Header or Banner-Footer.

Based on the description, it seems the chosen mode for my requirement should be Interstitial. However, I was unable to set the margin in the in-app html. Is this configuration in the in-app html or located elsewhere?