Question

<ComboBox Grid.Column="1" Grid.Row="1" ItemsSource="{Binding Locations}" SelectedItem="{Binding SelectedLocation}" Margin="5" MinWidth="125">
    <ComboBox.ItemContainerStyle>
        <Style TargetType="{x:Type ComboBoxItem}">
            <Style.Triggers>
                <Trigger Property="IsMouseOver" Value="true">
                    <Setter Property="Background" Value="Red" />
                    <Setter Property="BorderBrush" Value="Red" />
                    <Setter Property="BorderThickness" Value="2" />
                </Trigger>
            </Style.Triggers>
        </Style>
    </ComboBox.ItemContainerStyle>
</ComboBox>

Now the Border gets applied but the background color is still the standard windows selection color how do i overwrite it?

Answer 1

You need to override your SystemColors.HighlightBrushKey to override default brush for highlight. Add key to your combobox resources like this –

<ComboBox Grid.Column="1" Grid.Row="1" ItemsSource="{Binding Locations}" SelectedItem="{Binding SelectedLocation}" Margin="5" MinWidth="125">
  <ComboBox.Resources>
     <SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}" Color="Red"/>
  </ComboBox.Resources>
    <ComboBox.ItemContainerStyle>
        <Style TargetType="{x:Type ComboBoxItem}">
            <Style.Triggers>
                <Trigger Property="IsMouseOver" Value="true">
                    <Setter Property="BorderBrush" Value="Red" />
                    <Setter Property="BorderThickness" Value="2" />
                </Trigger>
            </Style.Triggers>
        </Style>
    </ComboBox.ItemContainerStyle>
</ComboBox>