How to make an imported Engage interaction transparent in Storylin

On old tried and true technique in Presenter was that users could make engage interactions have a transparent background and then allow the slide master to show through for a more seamless project.

With that being said, I dont see to see how to achieve this in Storyline and would like to continue this technique if possible. I dont see an option for slide master manipulation of this type, or where in the html output I could do a little post-publish fix-it.

Has anyone found a way to achieve this?

Thanks