Thank you for contacting us and for your question.
In your case you could benefit from our Dependency Property System by overriding the property values of the RadGroupBox's FillPrimitive that come from the currently set theme. This will allow you to prevent any changes implied by the theme since the property values set programmatically by you will have higher priority. Take a look at the following code snippet where I am defining a white solid background by setting properties of the RadGroupBox's FillPrimitive
this.radGroupBox1.GroupBoxElement.Content.Fill.GradientStyle = GradientStyles.Solid;
this.radGroupBox1.GroupBoxElement.Content.Fill.BackColor = Color.White;
I hope this helps. Do not hesitate to write back anytime you have further questions or need assistance.
the Telerik team
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