只垂直居中了，没有水平居中

抱歉，我的回答有误。可以将Grid中的Border和Rectangle包裹在一个StackPanel中，并将StackPanel的HorizontalAlignment属性设置为Center，如下所示：

<ItemsControl.ItemTemplate>
    <DataTemplate>
        <Grid VerticalAlignment="Center">
            <StackPanel HorizontalAlignment="Center">
                <Border Margin="{Binding Path=CharacterSpacing, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=local:CustomFont}}">
                    <TextBlock Text="{Binding}" />
                </Border>
                <Rectangle Height="1" Fill="{Binding Foreground, ElementName=itemCtrl}" Margin="0,0,0,5">
                    <Rectangle.Style>
                        <Style TargetType="Rectangle">
                            <Setter Property="Visibility" Value="Collapsed" />
                            <Style.Triggers>
                                <DataTrigger Binding="{Binding ShowUnderline, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=local:CustomFont}}" Value="True">
                                    <Setter Property="Visibility" Value="Visible" />
                                </DataTrigger>
                            </Style.Triggers>
                        </Style>
                    </Rectangle.Style>
                </Rectangle>
            </StackPanel>
        </Grid>
    </DataTemplate>
</ItemsControl.ItemTemplate>