When you change Background Styles within the Slide Master, the Background Style for all dependent Slide Layouts will change. However, it is not necessary for all your Slide Layouts to possess the same Background Style as the Slide Master — or even another Slide Layout. Each Layout can have its own independent Background Style, as you will learn in the rest of this tutorial.
Learn how to apply Background Styles to individual Slide Layouts in PowerPoint 2011 for Mac.

