var provider = new PdfFormatProvider();
//var document = provider.Import(File.ReadAllBytes(@"..\..\..\Systemvariablen.pdf"));
var document = provider.Import(File.ReadAllBytes(@"..\..\..\Simplified_TestForm1_cropped (1).pdf"));

foreach (var item in document.AcroForm.FormFields)
{ 
    var widget = item.Widgets.First() as VariableContentWidget;
     
    if (widget != null)
    {
        if (widget.Content.NormalContentSource == 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