OpenXML Excel C# Update Stylesheet and add new Style to existing Stylesheet - openxml

Question

I need to add another style to an existing stylesheet. I created this style using the OpenXML Productivity Tools. Unfortunately I can't manage to add this style to the existing stylesheet. The stylesheet is always overwritten. Do I need to adjust the counts (borders, fills, etc)? Or is there a better way?
void AppendStylesToStylesheet(WorkbookStylesPart workbookStylesPart)
{
var styleSheet = workbookStylesPart.Stylesheet;
var fonts1 = new Fonts { Count = 1U, KnownFonts = true };
var font1 = new Font();
font1.Append(new FontSize { Val = 11D });
font1.Append(new Color { Theme = 1U });
font1.Append(new FontName { Val = "Calibri" });
font1.Append(new FontFamilyNumbering { Val = 2 });
font1.Append(new FontScheme { Val = FontSchemeValues.Minor });
fonts1.Append(font1);
var fills1 = new Fills { Count = 3U };
var fill1 = new Fill();
fill1.Append(new PatternFill { PatternType = PatternValues.None });
var fill2 = new Fill();
fill2.Append(new PatternFill { PatternType = PatternValues.Gray125 });
var fill3 = new Fill();
var patternFill3 = new PatternFill { PatternType = PatternValues.Solid };
patternFill3.Append(new ForegroundColor { Theme = 0U, Tint = -4.9989318521683403E-2D });
patternFill3.Append(new BackgroundColor { Indexed = 64U });
fill3.Append(patternFill3);
fills1.Append(fill1);
fills1.Append(fill2);
fills1.Append(fill3);
var borders1 = new Borders { Count = 2U };
var border1 = new Border();
var leftBorder1 = new LeftBorder();
var rightBorder1 = new RightBorder();
var topBorder1 = new TopBorder();
var bottomBorder1 = new BottomBorder();
var diagonalBorder1 = new DiagonalBorder();
border1.Append(leftBorder1);
border1.Append(rightBorder1);
border1.Append(topBorder1);
border1.Append(bottomBorder1);
border1.Append(diagonalBorder1);
var border2 = new Border();
var leftBorder2 = new LeftBorder();
var rightBorder2 = new RightBorder();
var topBorder2 = new TopBorder { Style = BorderStyleValues.Dotted };
topBorder2.Append(new Color { Theme = 0U, Tint = -0.24994659260841701D });
var bottomBorder2 = new BottomBorder { Style = BorderStyleValues.Dotted };
bottomBorder2.Append(new Color { Theme = 0U, Tint = -0.24994659260841701D });
var diagonalBorder2 = new DiagonalBorder();
border2.Append(leftBorder2);
border2.Append(rightBorder2);
border2.Append(topBorder2);
border2.Append(bottomBorder2);
border2.Append(diagonalBorder2);
borders1.Append(border1);
borders1.Append(border2);
var cellStyleFormats1 = new CellStyleFormats { Count = 1U };
cellStyleFormats1.Append(new CellFormat { NumberFormatId = 0U, FontId = 0U, FillId = 0U, BorderId = 0U });
var cellFormats1 = new CellFormats { Count = 3U };
var cellFormat2 = new CellFormat { NumberFormatId = 0U, FontId = 0U, FillId = 0U, BorderId = 0U, FormatId = 0U };
var cellFormat3 = new CellFormat { NumberFormatId = 14U, FontId = 0U, FillId = 2U, BorderId = 1U, FormatId = 0U, ApplyNumberFormat = true, ApplyFill = true, ApplyBorder = true, ApplyAlignment = true };
var alignment1 = new Alignment { Vertical = VerticalAlignmentValues.Center };
cellFormat3.Append(alignment1);
var cellFormat4 = new CellFormat { NumberFormatId = 0U, FontId = 0U, FillId = 0U, BorderId = 0U, FormatId = 0U, ApplyAlignment = true };
var alignment2 = new Alignment { Vertical = VerticalAlignmentValues.Center };
cellFormat4.Append(alignment2);
cellFormats1.Append(cellFormat2);
cellFormats1.Append(cellFormat3);
cellFormats1.Append(cellFormat4);
var cellStyles1 = new CellStyles { Count = 1U };
var cellStyle1 = new CellStyle { Name = "Standard", FormatId = 0U, BuiltinId = 0U };
cellStyles1.Append(cellStyle1);
var differentialFormats1 = new DifferentialFormats { Count = 0U };
var tableStyles1 = new TableStyles { Count = 0U, DefaultTableStyle = "TableStyleMedium2", DefaultPivotStyle = "PivotStyleLight16" };
var colors1 = new Colors();
var mruColors1 = new MruColors();
var color4 = new Color { Rgb = "FFFFFFCC" };
mruColors1.Append(color4);
colors1.Append(mruColors1);
var stylesheetExtensionList1 = new StylesheetExtensionList();
var stylesheetExtension1 = new StylesheetExtension { Uri = "{EB79DEF2-80B8-43e5-95BD-54CBDDF9020C}" };
stylesheetExtension1.AddNamespaceDeclaration("x14", "http://schemas.microsoft.com/office/spreadsheetml/2009/9/main");
var slicerStyles1 = new X14.SlicerStyles { DefaultSlicerStyle = "SlicerStyleLight1" };
stylesheetExtension1.Append(slicerStyles1);
var stylesheetExtension2 = new StylesheetExtension { Uri = "{9260A510-F301-46a8-8635-F512D64BE5F5}" };
stylesheetExtension2.AddNamespaceDeclaration("x15", "http://schemas.microsoft.com/office/spreadsheetml/2010/11/main");
var timelineStyles1 = new X15.TimelineStyles { DefaultTimelineStyle = "TimeSlicerStyleLight1" };
stylesheetExtension2.Append(timelineStyles1);
stylesheetExtensionList1.Append(stylesheetExtension1);
stylesheetExtensionList1.Append(stylesheetExtension2);
styleSheet.Append(fonts1);
styleSheet.Append(fills1);
styleSheet.Append(borders1);
styleSheet.Append(cellStyleFormats1);
styleSheet.Append(cellFormats1);
styleSheet.Append(cellStyles1);
styleSheet.Append(differentialFormats1);
styleSheet.Append(tableStyles1);
styleSheet.Append(colors1);
styleSheet.Append(stylesheetExtensionList1);
styleSheet.Save();
}