The include method will not work in R markdown documents. Instead one should use, in a separate script, the write method to save the CSS file.

Linne$
  new()$
  rule(
    sel_tag("p"),
    color = "white",
    fontSize = 30
  )$
  write("style.css") # save 

Then use that file in the front matter of the R markdown file, for instance.

output:
  revealjs::revealjs_presentation:
    css: style.css