How can I assign a class to a div a runtime?

I have a div that displays a shadow around a box in a datalist.  The shadow can be a different color depending on the data.  After a Google search, I tried this:
<div runat="server" ID="GenderShadow">
(I tried it with and without the runat=server.)  In my code behind I have
If inmate.MALE_OR_FEMALE = "Female" Then
    GenderShadow.Attributes.Add("class", "p_FemaleBackground")
Else
    GenderShadow.Attributes.Add("class", "p_MaleBackground")
End If
I'm getting an error that Name GenderShadow has not been declared. How do I declare it?
Diane

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