Unfortunately, there is (as of PowerPoint 2013) no way to either set the default format for SmartArt or to use the Format Painter to format all shapes within a SmartArt graphic.
But, whenever a question like this arises, VBA macros and add-ins come to the rescue.
The very basic macro below takes the line and fill colour from either your selected shape or the default shape style if you don't select anything and applies it to each shape within the SmartArt graphic. If you don't know how to use a macro, take a look at these examples:
http://i-present.co.uk/category/blog/vba/
It's basic because there are literally hundreds of properties that a user could set such as fill gradients, pictures, textures, line colours, widths, dashes and effects such as reflection, glow etc.
I own a company called GMARK that specialises in PowerPoint add-in development (http://i-present.co.uk) and could create an add-in to do this if there was interest.
Sub SetSmartArtToDefaultShapeStyle()
Dim oSld As Slide
Dim oShpCheck As Shape, oShpSource As Shape, oShpNode
Dim oNode As SmartArtNode
Dim DeleteShape As Boolean
On Error GoTo errorhandler
Set oSld = ActivePresentation.Slides(ActiveWindow.View.Slide.SlideIndex)
If Not ActiveWindow.Selection.HasChildShapeRange Then
Set oShpSource = oSld.Shapes.AddShape(msoShapeRectangle, 0, 0, 10, 10)
DeleteShape = True
Else
Set oShpSource = ActiveWindow.Selection.ShapeRange(1)
End If
oShpSource.PickUp
For Each oShpCheck In oSld.Shapes ' As Shapes
With oShpCheck
If .HasSmartArt Then
For Each oNode In .SmartArt.Nodes
For Each oShpNode In oNode.Shapes ' As ShapeRange
With oShpNode
.Line.Visible = oShpSource.Line.Visible
.Fill.Visible = oShpSource.Line.Visible
If .Line.ForeColor.Type = msoColorTypeRGB Then _
.Line.ForeColor.RGB = oShpSource.Line.ForeColor.RGB
If .Line.ForeColor.Type = msoColorTypeScheme Then _
.Line.ForeColor.ObjectThemeColor = oShpSource.Line.ForeColor.ObjectThemeColor
If .Fill.ForeColor.Type = msoColorTypeRGB Then _
.Fill.ForeColor.RGB = oShpSource.Fill.ForeColor.RGB
If .Fill.ForeColor.Type = msoColorTypeScheme Then _
.Fill.ForeColor.ObjectThemeColor = oShpSource.Fill.ForeColor.ObjectThemeColor
End With
Next
Next
End If
End With
Next
If DeleteShape = True Then oShpSource.Delete
Exit Sub
errorhandler:
MsgBox "There was an error : " & Err.Number & " : " & Err.Description, vbCritical + vbOKOnly, "SmartArt Format by i-present.co.uk"
Err.Clear
If DeleteShape = True Then oShpSource.Delete
End Sub