I have the following code that adds a cell with values and data-type for that cell in OpenXML SDK:
Cell cell = InsertCellInWorksheet(column, row, workshee
The SpreadsheetDocument is structured as collection of WorkbookParts. One of these, the WorkbookStylesPart, contains all the styles used in the document. The WorkbookPart contains your Worksheets. In order to apply a style to a cell or range of cells, you need to set the StyleIndex property to the corresponding style in the WorkbookStylesPart.
This answer should help get you started: https://stackoverflow.com/a/11118442/741326