public class ThemeColor : IReadonlyThemeColor  

var workbook = Workbook.Create( "test.xlsx" )

 // Get the first worksheet. A workbook contains at least 1 worksheet.
 var cellWorksheet = workbook.Worksheets[ 0 ];

cellWorksheet.Cells[ "C18" ].Value = "Using theme color and tint";
cellWorksheet.Cells[ "C18" ].Style.Font = new Font() { ThemeColor = new ThemeColor( ThemeColorType.Accent2, -0.5d ) };

workbook.Save();