This was the missing piece of the puzzle! Switching to the RhinoWindows nuget package made the handler approach work. Now, we can override the appearance of individual elements like so:
Eto.Style.Add<Eto.Wpf.Forms.Controls.GroupBoxHandler>(null, h => {
h.Control.BorderThickness = new System.Windows.Thickness(2);
h.Control.BorderBrush = new System.Windows.Media.SolidColorBrush(System.Windows.Media.Color.FromRgb(255, 0, 255));
});
@curtisw, how would you approach developing for Mac, though? The whole idea behind using Eto was to have a cross-platform UI. I was hoping to use these style overrides only on Windows and fall back to defaults on OSX. This made sense while referencing RhinoCommon. Doesn’t switching to RhinoWindows invalidate this approach?