how to make background of combo box transparent and decide the color when mouse is over it

i find a solution : 
( <telerik:RadComboBox Margin="235,24,274,45" telerik:StyleManager.Theme="Metro" IsEditable="True" d:LayoutOverrides="Height" BorderBrush="{x:Null}" Background="{x:Null}">
<telerik:RadComboBoxItem Content="Item 1"/>
<telerik:RadComboBoxItem Content="Item 2"/>
<telerik:RadComboBoxItem Content="Item 3"/>)

but telerik:StyleManager.Theme is no set even i change it