Right Click on Slide Background and Select Fill Effects or More Colors
The More Colors option will allow you to select Standard Color or Custom Color as you wish .
The Fill Effects option will allow you to select Tabs such as Gradient, Texture, Pattern or Picture
Gradient
If you select this Tab you can add One
Color, Two Colors or Preset Colors to your Slide Background
Texture
This will allow you to add special
texture or other Texture to your slide background.
Pattern
The Pattern Tab will allow you to
add a Predefined Pattern
Picture
The Picture Tab will allow you to
select a picture from another location