You need to set appearance of your selection via styles.
<Window.Resources>
<Style TargetType="{x:Type ComboBoxItem}">
<Setter Property="Control.Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ComboBoxItem}">
<Border Background="{TemplateBinding Background}">
<Grid>
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition />
</Grid.RowDefinitions>
<Border Margin="2" Grid.Row="0" Background="Azure" />
<ContentPresenter />
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background" Value="Green" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Window.Resources>
This style will be automatically applied to any ComboBoxes within the window:
<StackPanel>
<ComboBox>
<ComboBoxItem>111</ComboBoxItem>
<ComboBoxItem>222</ComboBoxItem>
<ComboBoxItem>333</ComboBoxItem>
<ComboBoxItem>444</ComboBoxItem>
<ComboBoxItem>555</ComboBoxItem>
</ComboBox>
</StackPanel>
You will see it as follows:
UPD: In order to remove highlighting from selected item you need to modify system brushes which are actually used for these purposes. Just add two extra styles:
<SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}" Color="Transparent"/>
<SolidColorBrush x:Key="{x:Static SystemColors.HighlightTextBrushKey}" Color="Black"/>