Customizing ListBox and ListBoxItem

I want to do a small customization on my ListBox controls in the way that selected items look.  The default style is to make the selected item highlighted blue and surround it with a dashed border.  I have customized my ListBox a bit already, but I cannot figure out how to create my own style for the selected items.  Here is what I got so far:

<Style x:Key="HorizontalListBox"  TargetType="ListBox">
    <Setter Property="Template">
        ...some xaml...
    </Setter>
    <Setter Property="ItemsPanel">
        ...some xaml...
    </Setter>
    <Setter Property="ItemContainerStyle">
        <Setter.Value>
          & ...