Thank you for the additional details. The case where you use the mentioned theme and the settings are not applied is because in this theme the DrawFill property of the element is not set to true,
which is the needed value in order to allow the back color of the item to be painted. Please set this property as well and you should be good:
radPageView1.Pages.Item.BackColor = Color.Red;
radPageView1.Pages.Item.GradientStyle = GradientStyles.Solid;
Let me know how this works for you.
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