Unfortunately due to Silverlight 2.0 limitations this is currently not possible.
Let me elaborate a bit more -- when you choose to use non-default theme (Summer in this case), the theme is applied via applying styles to the Style properties of various chart elements (i.e. RadChart.Style, ChartTitle.Style, ChartArea.Style, ChartLegend.Style properties got set to value different than null). Silverlight 2.0 only allows for single style setting like this and in order to customize the legend background, you need to customize its template (i.e. you need to apply style that sets this template but you cannot apply style second time). Microsoft has addressed this limitation for the Silverlight 3.0 release and once it is out, you will be able to achieve the desired functionality.
the Telerik team