Silverlight transparent textbox background - silverlight

Question

I have a RadGridView (from Telerik) and I'm trying to be able to select the content of a cell.
So I have a cell template :
<telerik:GridViewDataColumn Header="{Binding Source={StaticResource ResourceStrings}, Path=ResourceStrings.Value}"
DataMemberBinding="{Binding Value}" UniqueName="Value" Width="2*">
<telerik:GridViewDataColumn.CellTemplate>
<DataTemplate>
<TextBox Text="{Binding Value}" IsReadOnly="True" Style="{StaticResource ReadOnlyStyle}" />
</DataTemplate>
</telerik:GridViewDataColumn.CellTemplate>
</telerik:GridViewDataColumn>
With my Style for the TextBox :
<Style x:Key="ReadOnlyStyle" TargetType="TextBox">
<Setter Property="BorderThickness" Value="0"/>
<Setter Property="Background" Value="Transparent"/>
<Setter Property="Padding" Value="0"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="TextBox">
<Grid x:Name="RootElement">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="MouseOver"/>
<VisualState x:Name="Disabled" />
<VisualState x:Name="ReadOnly"/>
</VisualStateGroup>
<VisualStateGroup x:Name="FocusStates">
<VisualState x:Name="Focused">
<Storyboard>
<DoubleAnimationUsingKeyFrames Storyboard.TargetName="FocusVisualElement" Storyboard.TargetProperty="Opacity">
<SplineDoubleKeyFrame KeyTime="0" Value="1"/>
</DoubleAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Unfocused">
<Storyboard>
<DoubleAnimationUsingKeyFrames Storyboard.TargetName="FocusVisualElement" Storyboard.TargetProperty="Opacity">
<SplineDoubleKeyFrame KeyTime="0" Value="0"/>
</DoubleAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Border x:Name="Border" Opacity="1" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" CornerRadius="1">
<Grid>
<Border x:Name="MouseOverBorder" BorderBrush="Transparent" BorderThickness="1">
<ScrollViewer x:Name="ContentElement" BorderThickness="0" IsTabStop="False" Padding="{TemplateBinding Padding}"/>
</Border>
</Grid>
</Border>
<Border x:Name="DisabledVisualElement" IsHitTestVisible="False" Opacity="0" Background="#A5F7F7F7" BorderBrush="#A5F7F7F7" BorderThickness="{TemplateBinding BorderThickness}"/>
<Border x:Name="FocusVisualElement" Margin="1" IsHitTestVisible="False" Opacity="0" BorderBrush="#FF6DBDD1" BorderThickness="{TemplateBinding BorderThickness}"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Here is the result :
How can I manage to remove this white background ?
Thanks
edit : In fact, We already had a textbox style that works with transparency and it works... I will post it later...