You can style it like anything else:
<Style TargetType="{x:Type ComboBox}" x:Key="HoverBox">
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background" Value="Blue" />
</Trigger>
</Style.Triggers>
</Style>
usage:
<ComboBox Style="{StaticResource HoverBox}" ... />
And at the top of your UserControl/ Window you have to place the style:
<UserControl...>
<UserControl.Resources>
<Style TargetType="{x:Type ComboBox}" x:Key="HoverBox">
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background" Value="Blue" />
</Trigger>
</Style.Triggers>
</Style>
</UserControl.Resources>
[CONTENT HERE]
</UserControl>
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