You can always fake a hard bevel with a reflected gradient on the stroke... something like this:
<Border Padding="5">
<Border.Background>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="#FF000000" Offset="0"/>
<GradientStop Color="#FF2B2B2B" Offset="1"/>
</LinearGradientBrush>
</Border.Background>
<Border BorderThickness="2,2,2,2" CornerRadius="10,10,10,10" Margin="20,20,20,20" Background="#FF838383">
<Border.BorderBrush>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0" MappingMode="RelativeToBoundingBox">
<GradientStop Color="#FF000000" Offset="0.474"/>
<GradientStop Color="#FFFFFFFF" Offset="1"/>
<GradientStop Color="#FFFFFFFF" Offset="0"/>
</LinearGradientBrush>
</Border.BorderBrush>
</Border>
</Border>
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