RadialGradientBrush Rectangle.Stroke

   
  

<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      WindowTitle="StackPanel vs. DockPanel">
  <Grid Width="175" Height="150">
    <Rectangle Grid.Row="1" Grid.Column="1"  Width="100" Height="100" 
      StrokeThickness="8" Margin="4">
      <Rectangle.Fill>
        <RadialGradientBrush GradientOrigin="0.5,0.5" Center="0.5,0.5" 
          RadiusX="0.5" RadiusY="0.5">
          <GradientStop Color="Red" Offset="0" />
          <GradientStop Color="Green" Offset="0.45" />
          <GradientStop Color="Yellow" Offset="0.85" />
        </RadialGradientBrush>
      </Rectangle.Fill>
      <Rectangle.Stroke>
        <RadialGradientBrush GradientOrigin="0.5,0.5" Center="0.5,0.5" 
          RadiusX="0.5" RadiusY="0.5">
          <GradientStop Color="Black" Offset="0.95" />
          <GradientStop Color="Gray" Offset="0.95" />
        </RadialGradientBrush>
      </Rectangle.Stroke>
    </Rectangle>
  </Grid>
</Page>