How do I change the appearance of the internal wires of a FemMesh?

You can derive a  MyFemMesh class overriding the DrawIsocurves() method as follows.
You can also control the color of the isocurves with fm.ElementEdgeColor.

public class MyFemMesh:FemMesh
    {
        protected override void DrawIsocurves(DrawParams data)
        {
            float prev = data.RenderContext.CurrentLineWidth;
            data.RenderContext.SetLineSize(5);
            base.DrawIsocurves(data);
            data.RenderContext.SetLineSize(prev);
        }

        protected override void DrawEdges(DrawParams data)
        {

        }
    } 

The picture below was obtained setting:

fm.ElementEdgeColor = Color.AntiqueWhite;