When adding a background to a slide, what is the difference between "Apply as Smart Media" and "Apply as Background"? What should guide your selection?
Welcome to the forums Sue!
According to this support article - https://support.faithlife.com/hc/en-us/articles/360007374611-How-Do-I-Change-the-Background-of-My-Slides-
Smart Media automatically adjusts the text formatting for the background you selected
So if you select a black background, for example, Apply as Smart Media will make sure that your text is a light, readable colour.