ok...I have done. Here is your answer. At first, this is not NPOI's fault, just because we don't know the thoughtof excel.
I write a simple code about it (Using C#)
sheet1.header.center="My title"; ==>It will print out the string using Excel 's default font.
and I change a little bit.
sheet1.header.center=" &B &14 &\"Arial\" My title"; ==>It will print out the string using Bold font ,Font size as 14 ,and Font name as Arial.
then you may find the clue.
I suggest you may find your answer by using VBA 's Programming thought. Good Luck.