Launch PowerPoint to create a new presentation, and you may typically see a single slide with a white background. Alternatively, if you open any of your existing presentations, the background of your slides may be in a different color depending upon the Theme that the presentation is based upon. You can always change this slide background to a picture, a solid color, a pattern, or even a gradient. However, considering all those options, explore the twelve background styles that PowerPoint offers for every presentation by default. These styles are all coordinated and also designed to work well as a set of complementary backgrounds.
Learn about different slide background styles available in PowerPoint 2011 for Mac.

