以下这个代码很长，是TreeViewItem的默认内置的xaml模板，我想修改一个细节，不知道具体做法。
下面这个代码很长（是TreeViewItem默认外观的模板代码），但是大家不用全部都看，只需要注意里面有一个控件：
……
<Border Name="Bd"
Grid.Column="1"
Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
……
我想修改这个Border，加入一行属性，就是：Grid.ColumnSpan=2，
具体怎么实现？求提供思路。或者如果简单的话，可否给出几行代码看看？
<!--================================================================= TreeViewItem ==================================================================--><Style x:Key="ExpandCollapseToggleStyle" TargetType="ToggleButton"> <Setter Property="Focusable" Value="False"/> <Setter Property="Template"> <Setter.Value> <ControlTemplate TargetType="ToggleButton"> <Grid Width="15" Height="13" Background="Transparent"> <Path x:Name="ExpandPath" HorizontalAlignment="Left" VerticalAlignment="Center" Margin="1,1,1,1" Fill="{StaticResource GlyphBrush}" Data="M 4 0 L 8 4 L 4 8 Z"/> </Grid> <ControlTemplate.Triggers> <Trigger Property="IsChecked" Value="True"> <Setter Property="Data" TargetName="ExpandPath" Value="M 0 4 L 8 4 L 4 8 Z"/> </Trigger> </ControlTemplate.Triggers> </ControlTemplate> </Setter.Value> </Setter></Style><Style x:Key="TreeViewItemFocusVisual"> <Setter Property="Control.Template"> <Setter.Value> <ControlTemplate> <Border> <Rectangle Margin="0,0,0,0" StrokeThickness="5" Stroke="Black" StrokeDashArray="1 2" Opacity="0"/> </Border> </ControlTemplate> </Setter.Value> </Setter></Style><Style x:Key="{x:Type TreeViewItem}" TargetType="{x:Type TreeViewItem}"> <Setter Property="Background" Value="Transparent"/> <Setter Property="HorizontalContentAlignment" Value="{Binding Path=HorizontalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"/> <Setter Property="VerticalContentAlignment" Value="{Binding Path=VerticalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}"/> <Setter Property="Padding" Value="1,0,0,0"/> <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/> <Setter Property="FocusVisualStyle" Value="{StaticResource TreeViewItemFocusVisual}"/> <Setter Property="Template"> <Setter.Value> <ControlTemplate TargetType="{x:Type TreeViewItem}"> <Grid> <Grid.ColumnDefinitions> <ColumnDefinition MinWidth="19" Width="Auto"/> <ColumnDefinition Width="Auto"/> <ColumnDefinition Width="*"/> </Grid.ColumnDefinitions> <Grid.RowDefinitions> <RowDefinition Height="Auto"/> <RowDefinition/> </Grid.RowDefinitions> <ToggleButton x:Name="Expander" Style="{StaticResource ExpandCollapseToggleStyle}" IsChecked="{Binding Path=IsExpanded, RelativeSource={RelativeSource TemplatedParent}}" ClickMode="Press"/> <Border Name="Bd" Grid.Column="1" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}"> <ContentPresenter x:Name="PART_Header" ContentSource="Header" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"/> </Border> <ItemsPresenter x:Name="ItemsHost" Grid.Row="1" Grid.Column="1" Grid.ColumnSpan="2"/> </Grid> <ControlTemplate.Triggers> <Trigger Property="IsExpanded" Value="false"> <Setter TargetName="ItemsHost" Property="Visibility" Value="Collapsed"/> </Trigger> <Trigger Property="HasItems" Value="false"> <Setter TargetName="Expander" Property="Visibility" Value="Hidden"/> </Trigger> <MultiTrigger> <MultiTrigger.Conditions> <Condition Property="HasHeader" Value="false"/> <Condition Property="Width" Value="Auto"/> </MultiTrigger.Conditions> <Setter TargetName="PART_Header" Property="MinWidth" Value="75"/> </MultiTrigger> <MultiTrigger> <MultiTrigger.Conditions> <Condition Property="HasHeader" Value="false"/> <Condition Property="Height" Value="Auto"/> </MultiTrigger.Conditions> <Setter TargetName="PART_Header" Property="MinHeight" Value="19"/> </MultiTrigger> <Trigger Property="IsSelected" Value="true"> <Setter TargetName="Bd" Property="Background" Value="{DynamicResource {x:Static SystemColors.HighlightBrushKey}}"/> <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.HighlightTextBrushKey}}"/> </Trigger> <MultiTrigger> <MultiTrigger.Conditions> <Condition Property="IsSelected" Value="true"/> <Condition Property="IsSelectionActive" Value="false"/> </MultiTrigger.Conditions> <Setter TargetName="Bd" Property="Background" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/> <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/> </MultiTrigger> <Trigger Property="IsEnabled" Value="false"> <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/> </Trigger> </ControlTemplate.Triggers> </ControlTemplate> </Setter.Value> </Setter></Style>