The telerik:StyleManager.Theme="Office_Silver" syntax would be the correct one if you were to apply a theme to the RadCoverFlow control.
However, RadCoverFlow control is an ItemsControl that simply presents its items. With very slight exceptions, basically it does not have a visual representation of theming. That means that the background of the control is not themable, it is not intended to be changed depending on the theme applied. If you wish to set a background to RadCoverFlow, use the straightforward way: Background="Gray", for example.
I hope this will shed some insight on the behavior of the control. If you wish to accomplish a certain visual effect, please share more details.
All the best,
the Telerik team
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