I agree with what Class Coder said. well if you want to see the implementation, here it is:
<Style x:Key="ControlListBoxItem" TargetType="ListBoxItem">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ListBoxItem">
<Border Name="ListItemBorder" CornerRadius="3" BorderThickness="1" BorderBrush="Black">
<ContentPresenter Content="{TemplateBinding Content}" ContentTemplate="{TemplateBinding ContentTemplate}"/>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter TargetName="ListItemBorder" Property="BorderBrush" Value="Blue"/>
<Setter TargetName="ListItemBorder" Property="TextBlock.FontSize" Value="12"/>
</Trigger>
<Trigger Property="IsSelected" Value="True">
<Setter TargetName="ListItemBorder" Property="TextBlock.Foreground" Value="White"/>
<Setter TargetName="ListItemBorder" Property="BorderBrush" Value="Red"/>
<Setter TargetName="ListItemBorder" Property="Background" Value="Green"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
<Setter Property="Margin" Value="1,1,0,1" />
</Style>
You can ignore the control template part if you don't want to style the listbox items.
Hope it helps! :)