PM> Install-Package Spire.XLS

using Spire.Xls;
using System.Drawing;

namespace AddWatermark
{
    class Program
    {
        static void Main(string[] args)
        {
            //initialize a new instance of the Workbook class and load an Excel file
            Workbook workbook = new Workbook();
            workbook.LoadFromFile(@"Sample.xlsx");

            //Create a font
            Font font = new Font("Arial", 40);
            //Define a text
            string watermark = "Confidential";

            //Loop through all worksheets in the file
            foreach (Worksheet sheet in workbook.Worksheets)
            {
                //Call the DrawWatermarkImage() method to create an image based on the text
                Image imgWtrmrk = DrawWatermarkImage(watermark, font, Color.LightCoral, Color.White, sheet.PageSetup.PageHeight, sheet.PageSetup.PageWidth);

                //Add the image to the left header section of each worksheet 
                sheet.PageSetup.LeftHeaderImage = imgWtrmrk;
                sheet.PageSetup.LeftHeader = "&G";

                //Change the view mode of the worksheet to page layout in order to view the watermark
                sheet.ViewMode = ViewMode.Layout;
            }

            //Save the result file
            workbook.SaveToFile("AddWatermark.xlsx", ExcelVersion.Version2013);
        }

        private static Image DrawWatermarkImage(string text, Font font, Color textColor, Color backColor, double height, double width)
        {
            //Create an image with specified width and height
            Image img = new Bitmap((int)width, (int)height);
            //Create a Graphics object from the image
            Graphics drawing = Graphics.FromImage(img);

            //Get the size of the text
            SizeF textSize = drawing.MeasureString(text, font);

            //Change the origin of the coordinate system by prepending the specified translation to the transformation matrix of the Graphics
            drawing.TranslateTransform(((int)width - textSize.Width) / 2, ((int)height - textSize.Height) / 2);

            //Apply rotation
            drawing.RotateTransform(-45);

            //Change the origin of the coordinate system by prepending the specified translation to the transformation matrix of the Graphics
            drawing.TranslateTransform(-((int)width - textSize.Width) / 2, -((int)height - textSize.Height) / 2);

            //Paint the background
            drawing.Clear(backColor);

            //Create a brush for the text
            Brush textBrush = new SolidBrush(textColor);

            //Draw the text onto the center position of the Graphics
            drawing.DrawString(text, font, textBrush, ((int)width - textSize.Width) / 2, ((int)height - textSize.Height) / 2);
            drawing.Save();
            return img;
        }
    }
}

Imports Spire.Xls
Imports System.Drawing

Namespace AddWatermark
    Friend Class Program
        Private Shared Sub Main(ByVal args As String())
            'initialize a new instance of the Workbook class and load an Excel file
            Dim workbook As Workbook = New Workbook()
            workbook.LoadFromFile("Sample.xlsx")

            'Create a font
            Dim font As Font = New Font("Arial", 40)
            'Define a text
            Dim watermark = "Confidential"

            'Loop through all worksheets in the file
            For Each sheet As Worksheet In workbook.Worksheets
                'Call the DrawWatermarkImage() method to create an image based on the text
                Dim imgWtrmrk As Image = DrawWatermarkImage(watermark, font, Color.LightCoral, Color.White, sheet.PageSetup.PageHeight, sheet.PageSetup.PageWidth)

                'Add the image to the left header section of each worksheet 
                sheet.PageSetup.LeftHeaderImage = imgWtrmrk
                sheet.PageSetup.LeftHeader = "&G"

                'Change the view mode of the worksheet to page layout in order to view the watermark
                sheet.ViewMode = ViewMode.Layout
            Next

            'Save the result file
            workbook.SaveToFile("AddWatermark.xlsx", ExcelVersion.Version2013)
        End Sub

        Private Shared Function DrawWatermarkImage(ByVal text As String, ByVal font As Font, ByVal textColor As Color, ByVal backColor As Color, ByVal height As Double, ByVal width As Double) As Image
            'Create an image with specified width and height
            Dim img As Image = New Bitmap(width, height)
            'Create a Graphics object from the image
            Dim drawing As Graphics = Graphics.FromImage(img)

            'Get the size of the text
            Dim textSize As SizeF = drawing.MeasureString(text, font)

            'Change the origin of the coordinate system by prepending the specified translation to the transformation matrix of the Graphics
            drawing.TranslateTransform((CInt(width) - textSize.Width) / 2, (CInt(height) - textSize.Height) / 2)

            'Apply rotation
            drawing.RotateTransform(-45)

            'Change the origin of the coordinate system by prepending the specified translation to the transformation matrix of the Graphics
            drawing.TranslateTransform(-(CInt(width) - textSize.Width) / 2, -(CInt(height) - textSize.Height) / 2)

            'Paint the background
            drawing.Clear(backColor)

            'Create a brush for the text
            Dim textBrush As Brush = New SolidBrush(textColor)

            'Draw the text onto the center position of the Graphics
            drawing.DrawString(text, font, textBrush, (CInt(width) - textSize.Width) / 2, (CInt(height) - textSize.Height) / 2)
            drawing.Save()
            Return img
        End Function
    End Class
End Namespace