in XML saved in Excel, it looks like this:
<Style ss:ID="s22">
<Borders>
<Border ss:Position="Left" ss:LineStyle="Continuous" ss:Weight="1"/>
<Border ss:Position="DiagonalLeft" ss:LineStyle="Continuous" ss:Weight="1"/>
...
, , XmlStyle, Border.Sides:
XmlStyle sBorder = new XmlStyle();
sBorder.Border.Color = Color.Black;
sBorder.Border.Weight = 1;
sBorder.Border.LineStyle = Borderline.Continuous;
sBorder.Border.Sides = BorderSides.DiagonalLeft;
sheet[x, y].Style = sBorder;
, , .