Styling

sheet["A2"].Style = sheet["A1"].Style;
sheet["A2"].Style.Font.Size = 10; //this will change font size for both cells

sheet["A2"].Style = sheet["A1"].Style.Clone();
sheet["A2"].Style.Font.Size = 10; //only cell A2 is affected

Color black = Color.Black; //used often
Color gray = new Color(0xaaaaaa); //hex rrggbb
Color yellow = new Color(255, 255, 0); //rgb
Color red = new Color("#f00"); //hex string
Color blue = new Color("#0000ff"); //hex string

var border = new CellBorder();
border.Left = new BorderEdge { Style = BorderStyle.Double, Color = Color.Black }; //apply double black border to left edge
border.All = new BorderEdge { Style = BorderStyle.Medium, Color = Color.Black }; //apply medium black border to left, right, top and bottom edge
border.Diagonal = new BorderEdge { Style = BorderStyle.Dashed, Color = Color.Black }; // diagonal border is supported also
border.DiagonalUp = true; //in this case you should set which diagonal to use
border.DiagonalDown = false; //in this case you should set which diagonal to use

var wb = new Workbook();
var sheet = wb.Sheets.AddSheet("Demo Sheet");
sheet["A1"].Style.Border = border; //apply prepared border to cell A1
sheet["B1"].Style.Border = border; //apply prepared border to cell B1
sheet["C1"].Style.Border = border; //apply prepared border to cell C1
sheet["C1"].Style.Border.Bottom = null; //watch out, you have removed bottom border on A1, A2 and A3

var range = sheet["A3", "E10"];

range.SetBorder(new BorderEdge { Style = BorderStyle.Thin, Color = Color.Black }); //border is easier to set on ranges

range.GetRow(0).SetOutsideBorder(new BorderEdge { Style = BorderStyle.Medium, Color = Color.Black }); //header may need a heavier border

range.SetOutsideBorder(new BorderEdge { Style = BorderStyle.Double, Color = Color.Black }); //use double border outside the table

wb.Save("Borders.xlsx");

var workbook = new Workbook();
var sheet = workbook.Sheets.AddSheet("Fonts");

workbook.DefaultFont.Name = "Arial"; //set default font on workbook level
workbook.DefaultFont.Size = 10; //set default font on workbook level            

var r = 0; //current row
var c = 1; //current column

++r; 
sheet[r, c].Value = "Normal";

++r;
sheet[r, c].Value = "Bold";
sheet[r, c].Style.Font.Bold = true;

++r;
sheet[r, c].Value = "Italic";
sheet[r, c].Style.Font.Italic = true;

++r;
sheet[r, c].Value = "Strike";
sheet[r, c].Style.Font.Strike = true;

++r;
sheet[r, c].Value = "Underline";
sheet[r, c].Style.Font.Underline = FontUnderline.Single;

++r;
sheet[r, c].Value = "Red";
sheet[r, c].Style.Font.Color = new Color("#f00");

++r;
sheet[r, c].Value = "20pt";
sheet[r, c].Style.Font.Size = 20;

++r;
sheet[r, c].Value = "This text will wrap into multiple lines.";
sheet[r, c].Style.Alignment.WrapText = true;

++r;
sheet[r, c].Value = "This text shrink to fit the cell.";
sheet[r, c].Style.Alignment.ShrinkToFit = true;

r = 0;
c = 3;

++r;
sheet[r, c].Value = "Left";
sheet[r, c].Style.Alignment.Horizontal = HorizontalAlignment.Left;

++r;
sheet[r, c].Value = "Right";
sheet[r, c].Style.Alignment.Horizontal = HorizontalAlignment.Right;

++r;
sheet[r, c].Value = "Center";
sheet[r, c].Style.Alignment.Horizontal = HorizontalAlignment.Center;

++r;
++r;
sheet[r, c].Value = "Indent 1";
sheet[r, c].Style.Alignment.Indent = 1; //3 spaces

++r;
sheet[r, c].Value = "Indent 2";
sheet[r, c].Style.Alignment.Indent = 2; //6 spaces

++r;
sheet[r, c].Value = "Indent 3";
sheet[r, c].Style.Alignment.Indent = 3; //9 spaces            

workbook.Save("Fonts.xlsx");

var wb = new Workbook();
var sheet = wb.Sheets.AddSheet("Numbers");

var r = 0; //current row
var c = 1; //current column

++r;
sheet[r, c].Value = 1;
sheet[r, c].Style.Format = null; //default format

++r;
sheet[r, c].Value = 1000;
sheet[r, c].Style.Format = "#,#0"; //thousands separator

++r;
sheet[r, c].Value = 1325.236;
sheet[r, c].Style.Format = "#,#0.00"; //two decimal digits

++r;
sheet[r, c].Value = DateTime.Today; //dates get date format set by default

++r;
sheet[r, c].Value = DateTime.Today;
sheet[r, c].Style.Format = "yyyy-mm-dd"; //custom date format

++r;
sheet[r, c].Value = DateTime.Now.TimeOfDay.ToString("hh\\:mm"); //formatting TimeSpan

++r;
sheet[r, c].Value = 0.25;
sheet[r, c].Style.Format = "0.0%";  //percentage

wb.Save("NumberFormat.xlsx");

var whiteFill = CellFill.Solid(new Color("#fff")); //white fill
var blackFill = new CellFill { Pattern = FillPattern.Solid, Foreground = Color.Black }; //make sure you set the foreground color

var wb = new Workbook();
var sheet = wb.Sheets.AddSheet("Chessboard");

var cells = sheet[1, 1, 8, 8];
cells.SetOutsideBorder(new BorderEdge { Color = Color.Black, Style = BorderStyle.Medium });

for (var i = 0; i < cells.Range.Width; i++)
    for (var j = 0; j < cells.Range.Height; j++)
        cells[j, i].Style.Fill = (i + j) % 2 == 0 ? whiteFill : blackFill;

var rows = cells.GetColumn(-1);
for (var i = 0; i < rows.Range.Height; i++)
{
    rows[i].Value = 8 - i;
    rows[i].Style.Alignment.Horizontal = HorizontalAlignment.Center;
    rows[i].Style.Alignment.Vertical = VerticalAlignment.Center;
}

var columns = cells.GetRow(8);
for (var i = 0; i < columns.Range.Width; i++)
{
    columns[i].Value = new string(new char[] { (Char)('A' + i) });
    columns[i].Style.Alignment.Horizontal = HorizontalAlignment.Center;
    columns[i].Style.Alignment.Vertical = VerticalAlignment.Center;
}

sheet.DefaultRowHeight = 40; //px
wb.Save("Fills.xlsx");