Set background color for selected items in a ListBox

You specify the SelectedItem Background for a ListBox with the SystemColors.HighlightBrushKey (focused) and SystemColors.ControlBrushKey (not focused)

<Style.Resources>
    <!-- Background of selected item when focussed -->
    <SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}"
                     Color="Green"/>
    <!-- Background of selected item when not focussed -->
    <SolidColorBrush x:Key="{x:Static SystemColors.ControlBrushKey}"
                     Color="LightGreen" />
</Style.Resources> 

<ListBox.Resources>
    <SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}">SteelBlue</SolidColorBrush>
</ListBox.Resources>

If you want this to apply out of focus as well you need to override an additional key:

<SolidColorBrush x:Key="{x:Static SystemColors.ControlBrushKey}">SteelBlue</SolidColorBrush>