How to change fill brush for node?

[ZedZipDev]

Thank you for the fork.
How can I change fill brush for some nodes programmatically?

 <!-- The fill brush for nodes. -->
 <LinearGradientBrush
     x:Key="nodeFillBrush"
     StartPoint="0,0"
     EndPoint="0,1"
     >
     <GradientStop Color="White" Offset="0" />
     <GradientStop Color="#709CFF" Offset="0.6" />
 </LinearGradientBrush>

For example, I need to display a node in critical state and I would display it in red.