Please, try moving you custom implicit style to the App.xaml resources as it won't get applied from Grid.Reosurces. Or, you can try setting it explicitly via the ItemContainerStyle property of the RadListBox control.
Hope this helps.
the Telerik team
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Attached you can find a sample project where the style is set explicitly via the ItemContainerStyle property of RadListBox and the rows are colored differently according to the set Background value.
Please, take a look at the project and let us know if you have any further inquiries.
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