I have done something like this using the following in a merged ResourceDictionary, it may help you:
<Style TargetType="ListBoxItem">
<Style.Resources>
<!--SelectedItem with focus-->
<SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}" Color="LightBlue" Opacity=".4"/>
<!--SelectedItem without focus-->
<SolidColorBrush x:Key="{x:Static SystemColors.InactiveSelectionHighlightBrushKey }" Color="LightBlue" Opacity=".4"/>
</Style.Resources>
</Style>