Programmatically format a date in an excel sheet using Office Open Xml SDK

I'm building an Excel xlsx spreadsheet using the office open XML SDK. I can add dates to the sheet by converting them to their "AO" date representation and setting the Cell Value to number. I can't, however, figure out how to add the dd-mm-yy formatting to the cell.

I've seen a lot of complicated examples where you have to create a stylesheet from scratch, add the format to it and then reference that format, but I figure there must be an easier way. I'm creating my sheet from a template, and Excel already has builtin formats/styles. IMO I should be able to just load the stylesheet from the excel file I'm using as a template and then apply the format. Can't figure out how to do it though.