my Datagrid is:
<data:DataGrid x:Name="dgSearchResults" Style="{StaticResource dgStyle}" Grid.Row="1" ColumnHeaderStyle="{StaticResource dgHeaderStyle}" >
I have defined styles in my App.xaml file:
<Style x:Key="dgStyle" TargetType="data:DataGrid">
<Setter Property="Background" Value="White"/>
<Setter Property="RowBackground" Value="#FFF6F6F6"/>
<Setter Property="Foreground" Value="#FF3A3B3B"/>
<Setter Property="FontFamily" Value="Verdana"/>
<Setter Property="FontSize" Value="13.333"/>
</Style>
EXCEPTION--> Invalid attribute value data:DataGrid for property TargetType. [Line: 61 Position: 43]