i coudnt find an exact solution to this problem. so adding my own solution that works
var cellAddress = new ExcelAddress(
<startingRow>,
<startingcolumn>,
<endingRow>,
<endingColumn>);
var cf = ws.ConditionalFormatting.AddGreaterThan(cellAddress);
cf.Formula = "0";
cf.Style.Fill.BackgroundColor.Color = Color.LightGreen;