public class Fill : Xceed.Workbooks.NET.StyleBase<IReadonlyFill>, IReadonlyFill  

using( var workbook = Workbook.Create( "test.xlsx" ) )
  {
    // Get the first worksheet. A workbook contains at least 1 worksheet.
    var cellWorksheet = workbook.Worksheets[ 0 ];

    // Set fill style for cell C12.
    cellWorksheet.Cells[ "C12" ].Style.Fill = new Fill() { PatternStyle = FillPattern.ThinDiagonalCrosshatch, PatternColor = Color.Green,  BackgroundColor = Color.Blue };
    // Set Cell C12 content.
    cellWorksheet.Cells[ "C12" ].Value = "Testing Fill Pattern";

    // Save workbook to disk.
    workbook.Save();
  }