In R bookdown how do I set the default code output background color

My R bookdown book with gitbook style renders with the code output in a gray box. I would like to change it to be a white box with a black border. I have found posts showing how to set the color for a specific block but not how to set the default. I think I need to add a css file like this:

--- 
title: x
author: clueless
date: "`r Sys.Date()`"
site: bookdown::bookdown_site
documentclass: book
output:
  bookdown::gitbook: 
    css: "style.css"
---

but I a don't know where to go from here. I know very little css. So clues for the clueless would be greatly appreciated.

Solution

Using only CSS

The previous answer is correct for the CSS rule but the selector must be more specific for a gitbook() format. This a matter of CSS specificity.

Using a browser inspector, you can get a sense of the selector you need to use to override default CSS style set by gitbook.

This simple css will replace background and add a border

.book .book-body .page-wrapper .page-inner section.normal pre {
    background: #fff;
    border: 1px solid #ddd;
    border-color: #000;
}

You put in as you do in style.css and use the css argument of bookdown::gitbook in index.Rmd or in _output.ymldepending of which you are using.

This will pick up the CSS.

I think you could also use !important to have the most specify on pre with

pre {
    background: #fff !important;
    border: 1px solid #ddd !important;
    border-color: #000 !important;
}

(Edit after comment above)

Using knitr feature

Another way would be to use class.source and class.output https://bookdown.org/yihui/rmarkdown-cookbook/chunk-styling.html

This would allow you to set a specific class for your output and target this class in your CSS.

it would require to apply the chunk option to all code chunk using

knitr::opts_chunk$set(class.output = "custom-output")

Then you could use pre.custom-output to target only the code chunk output part, using one of the two syntax above (full selectors or !important)