ControlTemplate TargetType

public class HexadecimalBox : ExtendedTextBox
{
    public static readonly DependencyProperty RedProperty = DependencyProperty.Register("Red", typeof(int), typeof(HexadecimalBox), new PropertyMetadata(0, Value_PropertyChanged));

    public int Red
    {
        get => (int)GetValue(RedProperty);
        set => SetValue(RedProperty, value);
    }

    static HexadecimalBox()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(HexadecimalBox), new FrameworkPropertyMetadata(typeof(HexadecimalBox)));
    }

    //Overrides, methods, events registered inside the OnApplyTemplate, etc.
}
    
<!--HexaDecimalBox Style-->
<Style TargetType="{x:Type n:HexadecimalBox}">
    <Setter Property="MaxLength" Value="9"/>

    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type n:HexadecimalBox}">
                <Border Width="{TemplateBinding Width}" Height="{TemplateBinding Height}" MinWidth="{TemplateBinding MinWidth}"
                        BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}"
                        Background="{TemplateBinding Background}" VerticalAlignment="{TemplateBinding VerticalAlignment}"
                        HorizontalAlignment="{TemplateBinding HorizontalAlignment}" SnapsToDevicePixels="True">
                    <ScrollViewer x:Name="PART_ContentHost" Focusable="False" HorizontalScrollBarVisibility="Hidden"
                                  VerticalScrollBarVisibility="Hidden" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>

    <Style.Triggers>
        <Trigger Property="IsEnabled" Value="False">
            <Setter Property="Opacity" Value="0.7"/>
        </Trigger>
    </Style.Triggers>
</Style>