ContentPresenter.ContentTemplate Property

<Style x:Key="{x:Type ComboBox}"
       TargetType="{x:Type ComboBox}">
  <Setter Property="SnapsToDevicePixels"
          Value="true" />
  <Setter Property="OverridesDefaultStyle"
          Value="true" />
  <Setter Property="ScrollViewer.HorizontalScrollBarVisibility"
          Value="Auto" />
  <Setter Property="ScrollViewer.VerticalScrollBarVisibility"
          Value="Auto" />
  <Setter Property="ScrollViewer.CanContentScroll"
          Value="true" />
  <Setter Property="MinWidth"
          Value="120" />
  <Setter Property="MinHeight"
          Value="20" />
  <Setter Property="Template">
    <Setter.Value>
      <ControlTemplate TargetType="{x:Type ComboBox}">
        <Grid>
          <VisualStateManager.VisualStateGroups>
            <VisualStateGroup x:Name="CommonStates">
              <VisualState x:Name="Normal" />
              <VisualState x:Name="MouseOver" />
              <VisualState x:Name="Disabled">
                <Storyboard>
                  <ColorAnimationUsingKeyFrames Storyboard.TargetName="PART_EditableTextBox"
                                                Storyboard.TargetProperty="(TextElement.Foreground).
                      (SolidColorBrush.Color)">
                    <EasingColorKeyFrame KeyTime="0"
                                         Value="{StaticResource DisabledForegroundColor}" />
                  </ColorAnimationUsingKeyFrames>
                </Storyboard>
              </VisualState>
            </VisualStateGroup>
            <VisualStateGroup x:Name="EditStates">
              <VisualState x:Name="Editable">
                <Storyboard>
                  <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.Visibility)"
                                                 Storyboard.TargetName="PART_EditableTextBox">
                    <DiscreteObjectKeyFrame KeyTime="0"
                                            Value="{x:Static Visibility.Visible}" />
                  </ObjectAnimationUsingKeyFrames>
                  <ObjectAnimationUsingKeyFrames
                      Storyboard.TargetProperty="(UIElement.Visibility)"
                                                 Storyboard.TargetName="ContentSite">
                    <DiscreteObjectKeyFrame KeyTime="0"
                                            Value="{x:Static Visibility.Hidden}" />
                  </ObjectAnimationUsingKeyFrames>
                </Storyboard>
              </VisualState>
              <VisualState x:Name="Uneditable" />
            </VisualStateGroup>
          </VisualStateManager.VisualStateGroups>
          <ToggleButton x:Name="ToggleButton"
                        Template="{StaticResource ComboBoxToggleButton}"
                        Grid.Column="2"
                        Focusable="false"
                        ClickMode="Press"
                        IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, 
              RelativeSource={RelativeSource TemplatedParent}}"/>
          <ContentPresenter x:Name="ContentSite"
                            IsHitTestVisible="False"
                            Content="{TemplateBinding SelectionBoxItem}"
                            ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
                            ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}"
                            Margin="3,3,23,3"
                            VerticalAlignment="Stretch"
                            HorizontalAlignment="Left">
          </ContentPresenter>
          <TextBox x:Name="PART_EditableTextBox"
                   Style="{x:Null}"
                   Template="{StaticResource ComboBoxTextBox}"
                   HorizontalAlignment="Left"
                   VerticalAlignment="Bottom"
                   Margin="3,3,23,3"
                   Focusable="True"
                   Background="Transparent"
                   Visibility="Hidden"
                   IsReadOnly="{TemplateBinding IsReadOnly}" />
          <Popup x:Name="Popup"
                 Placement="Bottom"
                 IsOpen="{TemplateBinding IsDropDownOpen}"
                 AllowsTransparency="True"
                 Focusable="False"
                 PopupAnimation="Slide">
            <Grid x:Name="DropDown"
                  SnapsToDevicePixels="True"
                  MinWidth="{TemplateBinding ActualWidth}"
                  MaxHeight="{TemplateBinding MaxDropDownHeight}">
              <Border x:Name="DropDownBorder"
                      BorderThickness="1">
                <Border.BorderBrush>
                  <SolidColorBrush Color="{DynamicResource BorderMediumColor}" />
                </Border.BorderBrush>
                <Border.Background>
                  <SolidColorBrush Color="{DynamicResource ControlLightColor}" />
                </Border.Background>
              </Border>
              <ScrollViewer Margin="4,6,4,6"
                            SnapsToDevicePixels="True">
                <StackPanel IsItemsHost="True"
                            KeyboardNavigation.DirectionalNavigation="Contained" />
              </ScrollViewer>
            </Grid>
          </Popup>
        </Grid>
        <ControlTemplate.Triggers>
          <Trigger Property="HasItems"
                   Value="false">
            <Setter TargetName="DropDownBorder"
                    Property="MinHeight"
                    Value="95" />
          </Trigger>
          <Trigger Property="IsGrouping"
                   Value="true">
            <Setter Property="ScrollViewer.CanContentScroll"
                    Value="false" />
          </Trigger>
          <Trigger SourceName="Popup"
                   Property="AllowsTransparency"
                   Value="true">
            <Setter TargetName="DropDownBorder"
                    Property="CornerRadius"
                    Value="4" />
            <Setter TargetName="DropDownBorder"
                    Property="Margin"
                    Value="0,2,0,0" />
          </Trigger>
        </ControlTemplate.Triggers>
      </ControlTemplate>
    </Setter.Value>
  </Setter>
</Style>