var provider = new PdfFormatProvider();
var document = provider.Import(File.ReadAllBytes(@"..\..\..\Systemvariablen.pdf"));

foreach (var item in document.AcroForm.FormFields)
{ 
    var widget = item.Widgets.First() as VariableContentWidget;
     
    if (widget != null)
    { 
        widget.AppearanceCharacteristics.Background = new RgbColor(255, 0, 0);
        widget.AppearanceCharacteristics.BorderColor = new RgbColor(0, 0, 255);
        widget.Border = new AnnotationBorder(2, AnnotationBorderStyle.Solid, null);
      
        widget.RecalculateContent(); 
    }
}

pdfViewer.Document = document;

public class MyPdfViewer : RadPdfViewer
{
    public PdfDocumentSource PdfDocumentSource
    {
        get { return (PdfDocumentSource)GetValue(PdfDocumentSourceProperty); }
        set { SetValue(PdfDocumentSourceProperty, value); }
    }

    public static readonly DependencyProperty PdfDocumentSourceProperty =
        DependencyProperty.Register("PdfDocumentSource", typeof(PdfDocumentSource), typeof(MyPdfViewer), new PropertyMetadata(new PropertyChangedCallback(OnDocumentSourceChanged)));

    private static void OnDocumentSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        MyPdfViewer viewer = (MyPdfViewer)d;

        viewer.SetDocumentSource((PdfDocumentSource)e.NewValue);
    }
}

<local:MyPdfViewer x:Name="pdfViewer" PdfDocumentSource="{Binding MyViewModelPropertyOfTypePdfDocumentSource}" />