WPF GroupBox Style not being applied on custom control - wpf

Question

I have created a custom control so simply draw the border of a geroupbox in red if there is an error. for some reason when I use the control the foreground colour doesn't get set correctly it defaults to white instead of green. My code is as follows:
public class ErrorGroupBox : GroupBox
{
static ErrorGroupBox()
{
}
public bool ErrorState
{
get { return (bool)GetValue(ErrorStateProperty); }
set { base.SetValue(ErrorStateProperty, value); }
}
public static readonly DependencyProperty ErrorStateProperty =
DependencyProperty.Register("ErrorState", typeof(bool), typeof(ErrorGroupBox), new UIPropertyMetadata(false));
}
The style is:
<Style x:Name="ErrorGroupBox" TargetType="local:ErrorGroupBox">
<Setter Property="Foreground" Value="Green"/>
<Setter Property="BorderThickness" Value="2"/>
<Setter Property="FontFamily" Value="Tahoma"/>
<Setter Property="FontSize" Value="13"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="GroupBox">
<Grid SnapsToDevicePixels="True" Margin="5">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="6"/>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="6"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
<RowDefinition Height="6"/>
</Grid.RowDefinitions>
<!--<Border Background="{TemplateBinding Background}" BorderBrush="Transparent"
BorderThickness="{TemplateBinding BorderThickness}"
CornerRadius="4" Grid.Column="1 " Grid.ColumnSpan="4"
Grid.Row="1" Grid.RowSpan="3" HorizontalAlignment="Stretch"/>-->
<!--The Title-->
<Border x:Name="Header" Grid.Column="2" Grid.RowSpan="2" HorizontalAlignment="Left"
Padding="3,1,3,0" VerticalAlignment="Stretch" >
<Border.Effect>
<DropShadowEffect BlurRadius="5" Direction="334"/>
</Border.Effect>
<ContentPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
Content="{TemplateBinding Header}"
ContentSource="Header"
ContentStringFormat="{TemplateBinding HeaderStringFormat}"
ContentTemplate="{TemplateBinding HeaderTemplate}"
RecognizesAccessKey="True" Height="Auto"
VerticalAlignment="Center"
HorizontalAlignment="Center"
OpacityMask="#FF3844BD" Margin="0,1,0,0">
</ContentPresenter>
</Border>
<ContentPresenter Margin="5"
SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
Content="{TemplateBinding Content}"
ContentStringFormat="{TemplateBinding ContentStringFormat}"
ContentTemplate="{TemplateBinding ContentTemplate}"
Grid.Column="1" Grid.ColumnSpan="2" Grid.Row="2" />
<Border BorderBrush="#BFBFBF" BorderThickness="{TemplateBinding BorderThickness}"
CornerRadius="4" Grid.ColumnSpan="3" Grid.Row="1" Grid.RowSpan="3" RenderTransformOrigin="0.5,0.5" Margin="0">
<Border.OpacityMask>
<MultiBinding ConverterParameter="7" UpdateSourceTrigger="Default">
<MultiBinding.Converter>
<BorderGapMaskConverter/>
</MultiBinding.Converter>
<Binding Path="ActualWidth" ElementName="Header"/>
<Binding Path="ActualWidth" RelativeSource="{RelativeSource Self}"/>
<Binding Path="ActualHeight" RelativeSource="{RelativeSource Self}"/>
</MultiBinding>
</Border.OpacityMask>
</Border>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="local:ErrorGroupBox.ErrorState" Value="True">
<Setter Property="Foreground" Value="Green"/>
</Trigger>
<Trigger Property="local:ErrorGroupBox.ErrorState" Value="False">
<Setter Property="Foreground" Value="Green"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
I use the control as follows:
<controls:ErrorGroupBox ErrorState="{Binding Error}" Header="Test Type" Margin="20,20,0,0" Name="groupBox1" HorizontalAlignment="Left" VerticalAlignment="Top" Width="226" Grid.Column="0">

Answer 1

If this is a default theme style you need to specify a different metadata type in your static constructor:
public class ErrorGroupBox : GroupBox
{
static ErrorGroupBox()
{
DefaultStyleKeyProperty.OverrideMetadata(typeof(ErrorGroupBox), new FrameworkPropertyMetadata(typeof(ErrorGroupBox )));
}