Custom Word formatting using R Markdown

Background A good while back (around 2 years as of this writing), I needed the feature to turn an R Markdown document into a Word document (that’s easy) and to apply custom styles to specific parts of the Word document (that was trickier). I found some good information in this RStudio article, but it didn’t quite address everything I was looking for. Specifically, I wanted to find a way to format certain parts of the R Markdown document in a specific style.