完全なcontrolTemplateを編集する必要があります。これは、自由に拡張できる小さな例です。まず、コンボボックスを開くToggleButtonのcontrolTemplate:
<ControlTemplate x:Key="ComboBoxToggleButton" TargetType="{x:Type ToggleButton}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition Width="15" />
</Grid.ColumnDefinitions>
<Border x:Name="Border" Grid.ColumnSpan="2" BorderThickness="1" />
<Path x:Name="Arrow"
Fill="Black"
SnapsToDevicePixels="True"
StrokeThickness="1"
VerticalAlignment="Center"
HorizontalAlignment="Center"
Grid.Column="1"
Data="M0,0 L0,3 L1,3 L1,4 L2,4 L2,5 L3,5 L3,6 L4,6 L4,5 L5,5 L5,4 L6,4 L6,3 L7,3 L7,0 L6,0 L6,1 L5,1 L5,2 L4,2 L4,3 L3,3 L3,2 L2,2 L2,1 L1,1 L1,0 L0,0 Z" />
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background" Value="Gray" TargetName="Border"/>
<Setter Property="BorderBrush" Value="DarkGray" TargetName="Border"/>
</Trigger>
<Trigger Property="IsFocused" Value="true">
<Setter Property="BorderBrush" Value="DarkGray" TargetName="Border"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
そして、これがこのテンプレートを使用するコンボボックスのスタイルです。
<Style TargetType="{x:Type ComboBox}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ComboBox}">
<Border CornerRadius="5" BorderThickness="2" BorderBrush="Red">
<Grid>
<ToggleButton x:Name="ToggleButton"
Grid.Column="2"
Focusable="false"
ClickMode="Press"
IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
Template="{StaticResource ComboBoxToggleButton}"/>
<ContentPresenter x:Name="ContentSite"
IsHitTestVisible="False"
Content="{TemplateBinding SelectionBoxItem}"
ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}"
Margin="3,3,21,3"
VerticalAlignment="Stretch"
HorizontalAlignment="Left">
</ContentPresenter>
<TextBox x:Name="PART_EditableTextBox"
Style="{x:Null}"
HorizontalAlignment="Left"
VerticalAlignment="Bottom"
Margin="3,3,21,3"
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"
Background="WhiteSmoke"
SnapsToDevicePixels="True"
MinWidth="{TemplateBinding ActualWidth}"
MaxHeight="{TemplateBinding MaxDropDownHeight}">
<Border x:Name="DropDownBorder" BorderThickness="1" Margin="0 0 2 0" />
<ScrollViewer SnapsToDevicePixels="True">
<StackPanel IsItemsHost="True" KeyboardNavigation.DirectionalNavigation="Contained" />
</ScrollViewer>
</Grid>
</Popup>
</Grid>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
それが役に立てば幸い。