<Style  TargetType="{x:Type DataGridCell}">

            <Style.Triggers>

                <Trigger  Property="IsSelected" Value="true">

                    <Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}" />

                    <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.HighlightTextBrushKey}}" />

                    <Setter Property="BorderBrush" Value="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}" />

                </Trigger>

                <MultiDataTrigger>

                    <MultiDataTrigger.Conditions>

                        <Condition Binding="{Binding IsSelected, RelativeSource={RelativeSource Self}}" Value="True" />

                        <Condition Binding="{Binding IsKeyboardFocusWithin, RelativeSource={RelativeSource AncestorType={x:Type DataGrid}, Mode=FindAncestor}}" Value="False" />

                    </MultiDataTrigger.Conditions>

                    <Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}" />

                    <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.HighlightTextBrushKey}}" />

                    <Setter Property="BorderBrush" Value="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}" />

                </MultiDataTrigger>

            </Style.Triggers>

        </Style>