Example:

//Instantiating a Workbook object
Workbook workbook = new Workbook();
Worksheet sheet = workbook.getWorksheets().get(0);
 
//Adds an empty conditional formatting
int index = sheet.getConditionalFormattings().add();
FormatConditionCollection fcs = sheet.getConditionalFormattings().get(index);
 
//Sets the conditional format range.
CellArea ca = CellArea.createCellArea(0, 0, 10, 10);
fcs.addArea(ca);
 
//Adds condition.
int conditionIndex = fcs.addCondition(FormatConditionType.TOP_10, OperatorType.NONE, null, null);   
//Sets the background color.
FormatCondition fc = fcs.get(conditionIndex);
fc.getStyle().setBackgroundColor(Color.getRed());
Top10 top10 = fc.getTop10();
//Set the Top N 
top10.setRank(5);  
//Saving the Excel file
workbook.save("output.xls");