The default highlight style is defined with the name “highlight_style”. But you can define your highlight style.
HPS.SegmentKey highlightStyle = HPS.Database.CreateRootSegment();
highlightStyle.GetMaterialMappingControl().
SetFaceColor(new RGBAColor(1, 0, 0)).
SetEdgeColor(new RGBAColor(1, 0, 0)).
SetVertexColor(new RGBAColor(1, 0, 0)).
SetMarkerColor(new RGBAColor(1, 0, 0)).
SetLineColor(new RGBAColor(1, 0, 0)).
SetTextColor(new RGBAColor(1, 0, 0));
HPS.PortfolioKey portfolio = GetSprocketsControl().Canvas.GetPortfolioKey();
portfolio.DefineNamedStyle("my_highlight_style", highlightStyle);
HPS.HighlightOptionsKit highlight_options = HPS.HighlightOptionsKit.GetDefault();
highlight_options.SetStyleName("my_highlight_style");