How to Override ride the Theme Brushes in code behind?

I need to update the value of a particular theme brush in code-behind. In XAML it is like this :

<Border HorizontalAlignment="Right" VerticalAlignment="Bottom" Margin="0,0,5,5" Width="36" Padding="2"
BorderThickness="1" BorderBrush="DarkGray" Background="#7FFCFCFC" CornerRadius="3"
x:Name="myBorder">
<Border.Resources>
<LinearGradientBrush x:Key="{x:Static themes:AssetResourceKeys.ButtonBackgroundDefaultedBrushKey}"
StartPoint="0,0"
EndPoint="0,1">
<LinearGradientBrush.GradientStops>
<GradientStop Color="Transparent"
Offset="0" />
<GradientStop Color="Transparent"
Offset="1" />
</LinearGradientBrush.GradientStops>
</LinearGradientBrush>
</Border.Resources>

</Border>

What is its equilavent in code behind?

Thanks,

Clint