When you want to change the default selection style on a ListBox it's fairly straight forward. All you need to do is create a control template and use triggers to set values when IsSelected is true.
<Style TargetType="ListBoxItem">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ListBoxItem">
<Border x:Name="ItemBorder"
BorderBrush="Black"
Background="LightGray"
BorderThickness="2"
CornerRadius="4"
Margin="3">
<ContentPresenter Margin="2" />
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsSelected" Value="True">
<Setter TargetName="ItemBorder"
Property="BorderBrush"
Value="Red"/>
</Trigger>
<Trigger Property="IsMouseOver" Value="True">
<Setter TargetName="ItemBorder"
Property="BorderBrush"
Value="Blue" />
</Trigger>
<MultiTrigger>
<MultiTrigger.Conditions>
<Condition Property="IsMouseOver" Value="False" />
<Condition Property="IsSelected" Value="False" />
</MultiTrigger.Conditions>
<Setter TargetName="ItemBorder"
Property="Opacity"
Value="0.50" />
</MultiTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<ListBox>
<ListBoxItem>One</ListBoxItem>
<ListBoxItem>Two</ListBoxItem>
<ListBoxItem>Three</ListBoxItem>
<ListBoxItem>Four</ListBoxItem>
<ListBoxItem>Five</ListBoxItem>
<ListBoxItem>Six</ListBoxItem>
<ListBoxItem>Seven</ListBoxItem>
<ListBoxItem>Eight</ListBoxItem>
</ListBox>