A quick look at Microsoft gives us the following to change the background
ActivePresentation.SlideMaster.Background.Fill.PresetGradient _
Style:=msoGradientHorizontal, Variant:=1, _
PresetGradientType:=msoGradientLateSunset
This example sets the background of slide one in the active presentation to a preset shade.
With ActivePresentation.Slides(1)
.FollowMasterBackground = False
.Background.Fill.PresetGradient Style:=msoGradientHorizontal, _
Variant:=1, PresetGradientType:=msoGradientLateSunset
End With