I think you need to add a row element to worksheet/sheetData, for example:
<row r="2" s="1" customFormat="1" />
and corresponding entries in the styles part. Look at fills, cellStyleXfs, cellXfs.
You can research these by looking them up in the spec, at http://webapp.docx4java.org/OnlineDemo/ecma376/SpreadsheetML/index.html
When you are ready to code, you can upload an xlsx which contains whatever it is you want, then click to have code generated for you.
In short, your best approach, once you have a basic understanding of Open XML parts and docx4j's JAXB based approach to them, is to use the code generator, and the spec, and beyond that, docx4j's source code.