How to change font-style

[Spandana-Gajangi]

I want to change header to bold and background color. How add font styles.

const workSheet = {};
workSheet.stylesXmlFile = "styles.xml";
workSheet.name = 'Invoices';
workSheet.cols = [{
  caption: 'Buyer',
  type: 'string',
  width:10
},
{
  caption: 'Invoice No',
  type: 'string',
  width: 15,
},

]
added stles.xml data from https://blogs.msdn.microsoft.com/brian_jones/2007/05/29/simple-spreadsheetml-file-part-3-formatting/
both files located in same folder.
but I get error,
can any one tell me what I'm doing wrong