How to style ComboBox Background on Mouse Hover?

Question

I have a Rounded Border that contains a ComboBox

Answer 1

Check your Previous Posts asking the same

Unable to style WPF ComboBox on Mouse Hover

How to change ComboBox's Background Property during Mouse Hover

EDIT

Use like this. Below is the default controltemplate of the combobox. Note the commented line of code for your requirement

   <Window.Resources>
        <!-- Fill Brushes -->

        <LinearGradientBrush x:Key="NormalBrush" StartPoint="0,0" EndPoint="0,1">
            <GradientBrush.GradientStops>
                <GradientStopCollection>
                    <GradientStop Color="#FFF" Offset="0.0"/>
                    <GradientStop Color="#CCC" Offset="1.0"/>
                </GradientStopCollection>
            </GradientBrush.GradientStops>
        </LinearGradientBrush>

        <LinearGradientBrush x:Key="HorizontalNormalBrush" StartPoint="0,0" EndPoint="1,0">
            <GradientBrush.GradientStops>
                <GradientStopCollection>
                    <GradientStop Color="#FFF" Offset="0.0"/>
                    <GradientStop Color="#CCC" Offset="1.0"/>
                </GradientStopCollection>
            </GradientBrush.GradientStops>
        </LinearGradientBrush>

        <LinearGradientBrush x:Key="LightBrush" StartPoint="0,0" EndPoint="0,1">
            <GradientBrush.GradientStops>
                <GradientStopCollection>
                    <GradientStop Color="#FFF" Offset="0.0"/>
                    <GradientStop Color="#EEE" Offset="1.0"/>
                </GradientStopCollection>
            </GradientBrush.GradientStops>
        </LinearGradientBrush>

        <LinearGradientBrush x:Key="HorizontalLightBrush" StartPoint="0,0" EndPoint="1,0">
            <GradientBrush.GradientStops>
                <GradientStopCollection>
                    <GradientStop Color="#FFF" Offset="0.0"/>
                    <GradientStop Color="#EEE" Offset="1.0"/>
                </GradientStopCollection>
            </GradientBrush.GradientStops>
        </LinearGradientBrush>

        <LinearGradientBrush x:Key="DarkBrush" StartPoint="0,0" EndPoint="0,1">
            <GradientBrush.GradientStops>
                <GradientStopCollection>
                    <GradientStop Color="#FFF" Offset="0.0"/>
                    <GradientStop Color="#AAA" Offset="1.0"/>
                </GradientStopCollection>
            </GradientBrush.GradientStops>
        </LinearGradientBrush>

        <LinearGradientBrush x:Key="PressedBrush" StartPoint="0,0" EndPoint="0,1">
            <GradientBrush.GradientStops>
                <GradientStopCollection>
                    <GradientStop Color="#BBB" Offset="0.0"/>
                    <GradientStop Color="#EEE" Offset="0.1"/>
                    <GradientStop Color="#EEE" Offset="0.9"/>
                    <GradientStop Color="#FFF" Offset="1.0"/>
                </GradientStopCollection>
            </GradientBrush.GradientStops>
        </LinearGradientBrush>

        <SolidColorBrush x:Key="DisabledForegroundBrush" Color="#888" />

        <SolidColorBrush x:Key="DisabledBackgroundBrush" Color="#EEE" />

        <SolidColorBrush x:Key="WindowBackgroundBrush" Color="#FFF" />

        <SolidColorBrush x:Key="SelectedBackgroundBrush" Color="#DDD" />

        <!-- Border Brushes -->

        <LinearGradientBrush x:Key="NormalBorderBrush" StartPoint="0,0" EndPoint="0,1">
            <GradientBrush.GradientStops>
                <GradientStopCollection>
                    <GradientStop Color="#CCC" Offset="0.0"/>
                    <GradientStop Color="#444" Offset="1.0"/>
                </GradientStopCollection>
            </GradientBrush.GradientStops>
        </LinearGradientBrush>

        <LinearGradientBrush x:Key="HorizontalNormalBorderBrush" StartPoint="0,0" EndPoint="1,0">
            <GradientBrush.GradientStops>
                <GradientStopCollection>
                    <GradientStop Color="#CCC" Offset="0.0"/>
                    <GradientStop Color="#444" Offset="1.0"/>
                </GradientStopCollection>
            </GradientBrush.GradientStops>
        </LinearGradientBrush>

        <LinearGradientBrush x:Key="DefaultedBorderBrush" StartPoint="0,0" EndPoint="0,1">
            <GradientBrush.GradientStops>
                <GradientStopCollection>
                    <GradientStop Color="#777" Offset="0.0"/>
                    <GradientStop Color="#000" Offset="1.0"/>
                </GradientStopCollection>
            </GradientBrush.GradientStops>
        </LinearGradientBrush>

        <LinearGradientBrush x:Key="PressedBorderBrush" StartPoint="0,0" EndPoint="0,1">
            <GradientBrush.GradientStops>
                <GradientStopCollection>
                    <GradientStop Color="#444" Offset="0.0"/>
                    <GradientStop Color="#888" Offset="1.0"/>
                </GradientStopCollection>
            </GradientBrush.GradientStops>
        </LinearGradientBrush>

        <SolidColorBrush x:Key="DisabledBorderBrush" Color="#AAA" />

        <SolidColorBrush x:Key="SolidBorderBrush" Color="#888" />

        <SolidColorBrush x:Key="LightBorderBrush" Color="#AAA" />

        <!-- Miscellaneous Brushes -->
        <SolidColorBrush x:Key="GlyphBrush" Color="#444" />

        <SolidColorBrush x:Key="LightColorBrush" Color="#DDD" />

        <ControlTemplate x:Key="ComboBoxToggleButton" TargetType="ToggleButton">
            <Grid>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition />
                    <ColumnDefinition Width="20" />
                </Grid.ColumnDefinitions>
                <Border
      x:Name="Border" 
      Grid.ColumnSpan="2"
      CornerRadius="2"
      Background="{StaticResource NormalBrush}"
      BorderBrush="{StaticResource NormalBorderBrush}"
      BorderThickness="1" />
                <Border 
      Grid.Column="0"
      CornerRadius="2,0,0,2" 
      Margin="1" 
      Background="{StaticResource WindowBackgroundBrush}" 
      BorderBrush="{StaticResource NormalBorderBrush}"
      BorderThickness="0,0,1,0" />
                <Path 
      x:Name="Arrow"
      Grid.Column="1"     
      Fill="{StaticResource GlyphBrush}"
      HorizontalAlignment="Center"
      VerticalAlignment="Center"
      Data="M 0 0 L 4 4 L 8 0 Z"/>
            </Grid>
            <ControlTemplate.Triggers>
                <!--<Trigger Property="ToggleButton.IsMouseOver" Value="true">
                    <Setter TargetName="Border" Property="Background" Value="{StaticResource DarkBrush}" />
                </Trigger>-->
                <Trigger Property="ToggleButton.IsChecked" Value="true">
                    <Setter TargetName="Border" Property="Background" Value="{StaticResource PressedBrush}" />
                </Trigger>
                <Trigger Property="IsEnabled" Value="False">
                    <Setter TargetName="Border" Property="Background" Value="{StaticResource DisabledBackgroundBrush}" />
                    <Setter TargetName="Border" Property="BorderBrush" Value="{StaticResource DisabledBorderBrush}" />
                    <Setter Property="Foreground" Value="{StaticResource DisabledForegroundBrush}"/>
                    <Setter TargetName="Arrow" Property="Fill" Value="{StaticResource DisabledForegroundBrush}" />
                </Trigger>
            </ControlTemplate.Triggers>
        </ControlTemplate>

        <ControlTemplate x:Key="ComboBoxTextBox" TargetType="TextBox">
            <Border x:Name="PART_ContentHost" Focusable="False" Background="{TemplateBinding Background}" />
        </ControlTemplate>

        <Style x:Key="{x:Type ComboBox}" TargetType="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="ComboBox">
                        <Grid>
                            <ToggleButton 
            Name="ToggleButton" 
            Template="{StaticResource ComboBoxToggleButton}" 
            Grid.Column="2" 
            Focusable="false"
            IsChecked="{Binding Path=IsDropDownOpen,Mode=TwoWay,RelativeSource={RelativeSource TemplatedParent}}"
            ClickMode="Press">
                            </ToggleButton>
                            <ContentPresenter
            Name="ContentSite"
            IsHitTestVisible="False" 
            Content="{TemplateBinding SelectionBoxItem}"
            ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
            ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}"
            Margin="3,3,23,3"
            VerticalAlignment="Center"
            HorizontalAlignment="Left" />
                            <TextBox x:Name="PART_EditableTextBox"
            Style="{x:Null}" 
            Template="{StaticResource ComboBoxTextBox}" 
            HorizontalAlignment="Left" 
            VerticalAlignment="Center" 
            Margin="3,3,23,3"
            Focusable="True" 
            Background="Transparent"
            Visibility="Hidden"
            IsReadOnly="{TemplateBinding IsReadOnly}"/>
                            <Popup 
            Name="Popup"
            Placement="Bottom"
            IsOpen="{TemplateBinding IsDropDownOpen}"
            AllowsTransparency="True" 
            Focusable="False"
            PopupAnimation="Slide">
                                <Grid 
              Name="DropDown"
              SnapsToDevicePixels="True"                
              MinWidth="{TemplateBinding ActualWidth}"
              MaxHeight="{TemplateBinding MaxDropDownHeight}">
                                    <Border 
                x:Name="DropDownBorder"
                Background="{StaticResource WindowBackgroundBrush}"
                BorderThickness="1"
                BorderBrush="{StaticResource SolidBorderBrush}"/>
                                    <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="IsEnabled" Value="false">
                                <Setter Property="Foreground" Value="{StaticResource DisabledForegroundBrush}"/>
                            </Trigger>
                            <Trigger Property="IsGrouping" Value="true">
                                <Setter Property="ScrollViewer.CanContentScroll" Value="false"/>
                            </Trigger>
                            <Trigger SourceName="Popup" Property="Popup.AllowsTransparency" Value="true">
                                <Setter TargetName="DropDownBorder" Property="CornerRadius" Value="4"/>
                                <Setter TargetName="DropDownBorder" Property="Margin" Value="0,2,0,0"/>
                            </Trigger>
                            <Trigger Property="IsEditable"
               Value="true">
                                <Setter Property="IsTabStop" Value="false"/>
                                <Setter TargetName="PART_EditableTextBox" Property="Visibility"    Value="Visible"/>
                                <Setter TargetName="ContentSite" Property="Visibility" Value="Hidden"/>
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
            <Style.Triggers>
            </Style.Triggers>
        </Style>

    </Window.Resources>