In a excel sheet you need to follow a specific order for create a style sheet. What you are not doing is to follow this order [As I provided in code example]. Easiest way to learn how actually style sheet is structured use Open XMl Productivity too  . You can analyse any Excel file for it's contents and also verify formatting too [A good tutorial]

As a support here i have provided a code for a basic style sheet for a workbook. This is the correct order you should have. [Here I have provided code for 2 styles, style index 0 the default and 1 a blot text with alignment]

 WorkbookStylesPart stylesheet = spreadsheet.WorkbookPart.AddNewPart<WorkbookStylesPart>();  Stylesheet workbookstylesheet = new Stylesheet();  // <Fonts>  Font font0 = new Font();  // Default font  Font font1 = new Font();  // Bold font  Bold bold = new Bold();  font1.Append(bold);  Fonts fonts = new Fonts();  // <APENDING Fonts>  fonts.Append(font0);  fonts.Append(font1);  // <Fills>  Fill fill0 = new Fill();  // Default fill  Fills fills = new Fills();  // <APENDING Fills>  fills.Append(fill0);  // <Borders>  Border border0 = new Border();  // Defualt border  Borders borders = new Borders();  // <APENDING Borders>  borders.Append(border0);  // <CellFormats>  CellFormat cellformat0 = new CellFormat() { FormatId = 0, FillId = 0, BorderId = 0 };   CellFormat cellformat1 = new CellFormat(new Alignment() { Horizontal HorizontalAlignmentValues.Center, Vertical = VerticalAlignmentValues.Center }) { FontId = 1 };   // <APENDING CellFormats>  CellFormats cellformats = new CellFormats();  cellformats.Append(cellformat0);  cellformats.Append(cellformat1);    // Append FONTS, FILLS , BORDERS & CellFormats to stylesheet <Preserve the ORDER>  workbookstylesheet.Append(fonts);  workbookstylesheet.Append(fills);  workbookstylesheet.Append(borders);  workbookstylesheet.Append(cellformats);  stylesheet.Stylesheet = workbookstylesheet;  stylesheet.Stylesheet.Save(); 

In a excel sheet you need to follow a specific order for create a style sheet. What you are not doing is to follow this order [As I provided in code example]. Easiest way to learn how actually style sheet is structured use Open XMl Productivity tool. You can analyse any Excel file for it's contents and also verify formatting too. [A good tutorial].

As a support here I have provided a code for a basic style sheet for a workbook. This is the correct order you should have. [Here I have provided code for 2 styles, style index 0 the default and 1 a blot text with alignment.]

WorkbookStylesPart stylesheet = spreadsheet.WorkbookPart .AddNewPart<WorkbookStylesPart>(); Stylesheet workbookstylesheet = new Stylesheet(); // <Fonts> Font font0 = new Font(); // Default font Font font1 = new Font(); // Bold font Bold bold = new Bold(); font1.Append(bold); Fonts fonts = new Fonts(); // <APENDING Fonts> fonts.Append(font0); fonts.Append(font1); // <Fills> Fill fill0 = new Fill(); // Default fill Fills fills = new Fills(); // <APENDING Fills> fills.Append(fill0); // <Borders> Border border0 = new Border(); // Defualt border Borders borders = new Borders(); // <APENDING Borders> borders.Append(border0); // <CellFormats> CellFormat cellformat0 = new CellFormat()  {  FormatId = 0,  FillId = 0,  BorderId = 0  };  Alignment alignment = new Alignment()  {  Horizontal = HorizontalAlignmentValues.Center,  Vertical = VerticalAlignmentValues.Center  }; CellFormat cellformat1 = new CellFormat(alignment) {  FontId = 1  }; // <APENDING CellFormats> CellFormats cellformats = new CellFormats(); cellformats.Append(cellformat0); cellformats.Append(cellformat1); // Append FONTS, FILLS , BORDERS & CellFormats to stylesheet <Preserve the ORDER> workbookstylesheet.Append(fonts); workbookstylesheet.Append(fills); workbookstylesheet.Append(borders); workbookstylesheet.Append(cellformats); stylesheet.Stylesheet = workbookstylesheet; stylesheet.Stylesheet.Save(); 

In a excel sheet you need to follow a specific order for create a style sheet. What you are not doing is to follow this order [As I provided in code example]. Easiest way to learn hoe actually style sheet is structured use Open XMl Productivity too . You can analyse any Excel file for it's contents and also verify formatting too [A good tutorial]

As a support here i have provided a code for a basic style sheet for a workbook. This is the correct order you should have.

 WorkbookStylesPart stylesheet = spreadsheet.WorkbookPart.AddNewPart<WorkbookStylesPart>(); Stylesheet workbookstylesheet = new Stylesheet(); // <Fonts> Font font0 = new Font(); // Default font Font font1 = new Font(); // Bold font Bold bold = new Bold(); font1.Append(bold); Fonts fonts = new Fonts(); // <APENDING Fonts> fonts.Append(font0); fonts.Append(font1); // <Fills> Fill fill0 = new Fill(); // Default fill Fills fills = new Fills(); // <APENDING Fills> fills.Append(fill0); // <Borders> Border border0 = new Border(); // Defualt border Borders borders = new Borders(); // <APENDING Borders> borders.Append(border0); // <CellFormats> CellFormat cellformat0 = new CellFormat() { FormatId = 0, FillId = 0, BorderId = 0 }; CellFormat cellformat1 = new CellFormat(new Alignment() { Horizontal HorizontalAlignmentValues.Center, Vertical = VerticalAlignmentValues.Center }) { FontId = 1 }; // <APENDING CellFormats> CellFormats cellformats = new CellFormats(); cellformats.Append(cellformat0); cellformats.Append(cellformat1); // Append FONTS, FILLS , BORDERS & CellFormats to stylesheet <Preserve the ORDER> workbookstylesheet.Append(fonts); workbookstylesheet.Append(fills); workbookstylesheet.Append(borders); workbookstylesheet.Append(cellformats); stylesheet.Stylesheet = workbookstylesheet; stylesheet.Stylesheet.Save(); 

Note - In your specific case you have omitted much needed Fills and Borders.

In a excel sheet you need to follow a specific order for create a style sheet. What you are not doing is to follow this order [As I provided in code example]. Easiest way to learn how actually style sheet is structured use Open XMl Productivity too . You can analyse any Excel file for it's contents and also verify formatting too [A good tutorial]

As a support here i have provided a code for a basic style sheet for a workbook. This is the correct order you should have. [Here I have provided code for 2 styles, style index 0 the default and 1 a blot text with alignment]

 WorkbookStylesPart stylesheet = spreadsheet.WorkbookPart.AddNewPart<WorkbookStylesPart>(); Stylesheet workbookstylesheet = new Stylesheet(); // <Fonts> Font font0 = new Font(); // Default font Font font1 = new Font(); // Bold font Bold bold = new Bold(); font1.Append(bold); Fonts fonts = new Fonts(); // <APENDING Fonts> fonts.Append(font0); fonts.Append(font1); // <Fills> Fill fill0 = new Fill(); // Default fill Fills fills = new Fills(); // <APENDING Fills> fills.Append(fill0); // <Borders> Border border0 = new Border(); // Defualt border Borders borders = new Borders(); // <APENDING Borders> borders.Append(border0); // <CellFormats> CellFormat cellformat0 = new CellFormat() { FormatId = 0, FillId = 0, BorderId = 0 }; CellFormat cellformat1 = new CellFormat(new Alignment() { Horizontal HorizontalAlignmentValues.Center, Vertical = VerticalAlignmentValues.Center }) { FontId = 1 }; // <APENDING CellFormats> CellFormats cellformats = new CellFormats(); cellformats.Append(cellformat0); cellformats.Append(cellformat1); // Append FONTS, FILLS , BORDERS & CellFormats to stylesheet <Preserve the ORDER> workbookstylesheet.Append(fonts); workbookstylesheet.Append(fills); workbookstylesheet.Append(borders); workbookstylesheet.Append(cellformats); stylesheet.Stylesheet = workbookstylesheet; stylesheet.Stylesheet.Save(); 

Note - In your specific case you have omitted much needed Fills and Borders.