In the AlternationIndex trigger, why doesn't my background property work? - wpf

Question

code show as below：
This is part of my custom datagrid appearance.
<Style x:Key="DatagridStyle_1" TargetType="DataGrid">
<!--<Setter Property="ColumnHeaderStyle" Value="{DynamicResource Datagrid_HearderStyle_1}"/>
<Setter Property="CellStyle" Value="{DynamicResource Datagrid_CellStyle_1}"/>-->
<Setter Property="RowStyle" Value="{DynamicResource ConTemplate_DgRow_1}"/>
<Setter Property="GridLinesVisibility" Value="None" />
<Setter Property="AlternationCount" Value="2"/>
<!--<Setter Property="Template" Value="{DynamicResource DataGridTemplate1211}"/>-->
<Style x:Key="ConTemplate_DgRow_1" TargetType="{x:Type DataGridRow}">
<Setter Property="Background" Value="White"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="DataGridRow">
<Grid SnapsToDevicePixels="True" Background="Transparent">
<Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="1"
>
</Border>
<DataGridCellsPresenter Grid.Column="1"
ItemsPanel="{TemplateBinding ItemsPanel}"
/>
</Grid>
<!--<Border BorderThickness="1" Background="LightGreen" SnapsToDevicePixels="True">
<DataGridCellsPresenter Grid.Column="1" ItemsPanel="{TemplateBinding ItemsPanel}"/>
</Border>-->
<ControlTemplate.Triggers>
<!--<Trigger Property="IsSelected" Value="true">
<Setter Property="BorderBrush" Value="#00BCD4"/>
</Trigger>-->
<Trigger Property="IsMouseOver" Value="true">
<Setter Property="BorderBrush" Value="#00BCD4"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
<Trigger Property="AlternationIndex" Value="0">
<Setter Property="Background" Value="DarkBlue"/>
<Setter Property="Foreground" Value="LightGreen"/>
</Trigger>
</Style.Triggers>
</Style>
this part doesn't work,but the other part
<Setter Property="Foreground" Value="LightGreen"/> works fine
In the above code, I set the AlternationIndex trigger in the penultimate part of the code. In the same trigger, the Foreground property has already taken effect, but the Background property has never taken effect. I have modified the cell template and datagrid template before, and set their background color to transparent, but it has no effect.
I'd like to understand why this is the case and how to make setting the background color work.