Apply a template to your ListPicker
<toolkit:ListPicker Template="{StaticResource ListPickerControlTemplate1}">
Where ListPickerControlTemplate1 is a control template with Highlighted VisualState in which you can change the border brush, background color etc. In this case, I'm using a yellow background
<ControlTemplate x:Key="ListPickerControlTemplate1" TargetType="toolkit:ListPicker">
<StackPanel>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="PickerStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="Highlighted">
<Storyboard>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetName="UserControl"
Storyboard.TargetProperty="Foreground"
Duration="0">
<DiscreteObjectKeyFrame
Value="{StaticResource PhoneForegroundBrush}"
KeyTime="0"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetName="Border"
Storyboard.TargetProperty="Background"
Duration="0">
<DiscreteObjectKeyFrame
Value="{StaticResource PhoneBackgroundBrush}"
KeyTime="0"/>
</ObjectAnimationUsingKeyFrames>
<ObjectAnimationUsingKeyFrames
Storyboard.TargetName="Border"
Storyboard.TargetProperty="BorderBrush"
Duration="0">
<DiscreteObjectKeyFrame
Value="Yellow"
KeyTime="0"/>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
....
...........
</ControlTemplate>
I removed the ContentControl and some other parts from the control template because it's not so relevant.