Criei uma classe de caixa de texto personalizada para validar a entrada do usuário para permitir apenas valores hexadecimais e usei essa nova caixa de texto (HexTextBox) no xaml. Funciona bem, mas o HexTextBox perde todo o estilo do Mahapps, incluindo o esquema de cores e o TextBoxHelper. Você sabe como usar esse TexBox estendido e manter o estilo?

HexTextBox:

    public class HexTextBox : TextBox
    {
    public HexTextBox()
    {

    }
    /// <summary>
    /// Raise when a keyboard key is pressed.
    /// </summary>
    /// <param name="e">The event args.</param>
    protected override void OnPreviewKeyDown(KeyEventArgs e)
    {
        if (e.Key == Key.Space)
        {
            e.Handled = true;
        }

        base.OnPreviewKeyDown(e);
    }

    /// <summary>
    /// Raise when a text will be inputed in the text box object.
    /// </summary>
    /// <param name="e">The event args.</param>
    protected override void OnTextInput(TextCompositionEventArgs e)
    {
        int hexNumber;

        e.Handled = !int.TryParse(e.Text, NumberStyles.HexNumber, CultureInfo.CurrentCulture, out hexNumber);

        base.OnTextInput(e);
    }
}

Window.xaml

<UserControl
...
    xmlns:CoreWPF="clr-namespace:CoreWPF;assembly=CoreWPF" 
...>

<CoreWPF:HexTextBox 
        Text="{Binding DataXor1, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
        Grid.Column="2" Grid.Row="0"
        controls:TextBoxHelper.ClearTextButton="True"
        Height="26"
        TextWrapping="Wrap" 
        CharacterCasing="Upper"
        VerticalAlignment="Center"/>

Desde já, obrigado!