title: Changing chunk background color in RMarkdown
body: <p>I would like to have a certain code chunk highlighted in a different color (e.g. red) to indicate that it is bad practice. If I was using <code>.Rnw</code>, I could add the chunk option <code>background = 'red'</code> and get what I want, but this does not seem to work in <code>.Rmd</code>. My guess is that I need to make a custom css stylesheet (though what the selector would be, I don't know), and maybe also create a custom hook. I'd like it to be on a per-chunk basis, not an overall change for the entire document.</p>