public class Theme : WorkbookElement 

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

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

// Modify Workbook theme name and colors.
workbook.Theme.Name = "My Theme";
workbook.Theme.Colors[ ThemeColorType.Accent1 ].Color = Color.Red;
workbook.Theme.Colors[ ThemeColorType.Text1 ].Color = Color.Green;

// Add a title using the new Theme default Text1 color.
cellWorksheet.Cells[ "B1" ].Value = "Modify theme";
cellWorksheet.Cells[ "B1" ].Style.Font = new Font() { Bold = true, Size = 15.5d };

cellWorksheet.Cells[ "C5" ].Value = "Using new Accent1 color";
cellWorksheet.Cells[ "C5" ].Style.Font = new Font() { ThemeColor = new ThemeColor( ThemeColorType.Accent1 ) };

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