<LinearGradientBrush StartPoint="0,0" EndPoint="0,1">
<GradientStop Color="#FFF3F3F3" Offset="0"/>
<GradientStop Color="#FFEBEBEB" Offset="0.5"/>
<GradientStop Color="#FFDDDDDD" Offset="1"/>
</LinearGradientBrush>
For future reference you can find such information by either using Blend or a utility like
Snoop[
^]