$ pandoc --reference-docx=style.docx test.md -o test.docx
A little bit of man:
--reference-docx=FILE Use the specified file as a style reference in producing a docx file. For best results, the reference docx should be a modified version of a docx file produced using pandoc. The contents of the reference docx are ignored, but its stylesheets and document properties (including margins, page size, header, and footer) are used in the new docx. If