TL;DR: If you want to customize your tables and you don’t want to start with Pandoc’s default reference.docx template, you need to create your own new table style, name it Table, and modify that style to do whatever you want tables to do. I’m helping automate a documentation process at work, and the last step involves converting a Markdown file to .docx. The team we’re helping sent over a reference file with all their custom “styles” (in quotes here because none of the formatting they want to use is a style in the Word sense of the word).