I have Applied Expression_Dark As a theme for RadtreeView and also Windows8 theme for entire application now the problem is I wan't different color when the element gets selected or gets the focus so let me what to change.
<telerik:RadTreeView x:Name="DashBoardChartTree" telerik:StyleManager.Theme="Expression_Dark" TabIndex="7" Margin="5" KeyboardNavigation.TabNavigation="Continue" IsLineEnabled="True" SelectedItem="{Binding SelectedItem,Mode=TwoWay}" ItemsSource="{Binding DashBoards}" Grid.Row="3"
ItemTemplate="{StaticResource TreeDashBoard}" PreviewDragEnded="radTreeView_PreviewDragEnded" IsDragDropEnabled="True" SelectedValuePath="{Binding DashBoard}" SelectionChanged="DashBoardChartTree_SelectionChanged_1" ExpanderStyle="{StaticResource ExpanderStyle}">
<telerik:RadTreeView x:Name="DashBoardChartTree" telerik:StyleManager.Theme="Expression_Dark" TabIndex="7" Margin="5" KeyboardNavigation.TabNavigation="Continue" IsLineEnabled="True" SelectedItem="{Binding SelectedItem,Mode=TwoWay}" ItemsSource="{Binding DashBoards}" Grid.Row="3"
ItemTemplate="{StaticResource TreeDashBoard}" PreviewDragEnded="radTreeView_PreviewDragEnded" IsDragDropEnabled="True" SelectedValuePath="{Binding DashBoard}" SelectionChanged="DashBoardChartTree_SelectionChanged_1" ExpanderStyle="{StaticResource ExpanderStyle}">