Sorry for delaying our response.
That, actually, is the expected behavior when we talk about a control similar to ContentControl. Currently, the Background property of the RadBusyIndicator sets the background of the centered rectangle that shows progress. If you look for a way to apply a background of the whole RadBusyIndicator, you can use the following workaround:
your content here
Hope this explains the matter. Let us know in case you have further queries.
All the best,
the Telerik team
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