The themes are not containing any styles for RadDiagram and you cannot select a theme for this control. By default, the diagram has a transparent background. Here is how you can set the background and the grid color:
radDiagram1.BackColor = Color.LightBlue;
radDiagram1.BackgroundGrid.LineStroke = Brushes.Crimson;
I hope this will be useful. Let me know if you have additional questions.
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