WPF overrided datagrid how to set DatagridCell Selected style

See more:

WPF

DataGrid

I have overrides datagrid. Below are my code and xaml. how can i set style to change the datagridcell background colour when the cell is selected?

public class DataGridExt : DataGrid
    {
        public DataGridExt()
        { this.AutoGeneratedColumns += new EventHandler(DataGrid_AutoGeneratedColumns);
        }
    }

<vw:DataGridExt Grid.Row="1" Background="White" AutoGenerateColumns="True" IsReadOnly="True"
    ItemsSource="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Value,  Mode=OneWay, IsAsync=True}"
    AlternatingRowBackground="LightGray"CanUserResizeRows="False" ClipboardCopyMode="IncludeHeader"
    CanUserSortColumns="True" CanUserAddRows="False" SelectionMode="Extended" />