Code block and output background color and border color in Rmarkdown Beamer

Question

I am trying to highlight (background colors and border) a code block in R Markdown with the output format being beamer presentation. I use the metropolis theme. The sample code is as follows

---
title: "Introduction"
author: ""
date: '`r Sys.Date()`'
output:
  beamer_presentation:
    keep_tex: yes
    theme: metropolis
    latex_engine: xelatex
    slide_level: 2
    incremental: no
fontsize: 12pt
classoption: compress
header-includes:
  \setbeamercolor{frametitle}{bg=darkgray}
    \hypersetup{colorlinks,citecolor=orange,filecolor=red,linkcolor=brown,urlcolor=blue}
---


```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```

```{css, echo=FALSE}
.normal-code {
  background-color: darkgray;
  border: 3px solid red;
  font-weight: bold;
}
```
## First Slide

```{r class.source="normal-code",warning=FALSE,message=FALSE,eval=TRUE,echo=TRUE}
# Load R package
library(tidyverse)
```

Currently, there is neither a change of background color nor a border around the code.

Answer 1

CSS won't have any effect on beamer output. You need latex-oriented solutions.

I have proposed two solutions; the first one is simpler whereas the second one gets a bit complicated (but looks better and customizable).

Approach One

First one is really simple. Just redefine the shadecolor with whatever color you like using, \definecolor{shadecolor}{RGB}{225, 225, 225} which changes the bg color of Shaded environemnt and use chunk opiton class.output="shaded"

Note: Shaded is used by rmarkdown for code blocks and if you use class.output chunk option, Shaded used for code output too.

---
title: "Introduction"
author: ""
date: '`r Sys.Date()`'
output:
  beamer_presentation:
    keep_tex: yes
    theme: metropolis
    latex_engine: xelatex
    slide_level: 2
    incremental: no
fontsize: 12pt
classoption: compress
header-includes:
  - \setbeamercolor{frametitle}{bg=darkgray}
  - \hypersetup{colorlinks,citecolor=orange,filecolor=red,linkcolor=brown,urlcolor=blue}
---

## First Slide


```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```

\definecolor{shadecolor}{RGB}{225, 225, 225}

```{r class.output="shaded"}
a <- c(1,2,3,4,5)
b <- c(1,2,3,4,5)
df <- data.frame(a, b)

# take a look at our data frame
df
```

---

---

Approach Two

Now for more customization, we may try with tcolorbox latex package. So in this second approach, I have done the followings,

• Firstly, defined a colored box using \newtcolorbox along with options for bg color and border color (Refer to package docs for the details)

• And then secondly, modified the source and output knitr hooks, so that source code and output is wrapped within the previously defined color box.

---
title: "Introduction"
author: ""
date: '`r Sys.Date()`'
output:
  beamer_presentation:
    keep_tex: yes
    theme: metropolis
    latex_engine: xelatex
    slide_level: 2
    incremental: no
fontsize: 12pt
classoption: compress
header-includes:
  - \setbeamercolor{frametitle}{bg=darkgray}
  - \hypersetup{colorlinks,citecolor=orange,filecolor=red,linkcolor=brown,urlcolor=blue}
  - \usepackage{tcolorbox}
---

## First Slide


```{=latex}

\definecolor{shadecolor}{RGB}{249, 247, 223}

\newtcolorbox{codebox}{
    colback=shadecolor,
    colframe=orange,
    boxsep=2pt,
    arc=2pt}
```


```{r setup, include=FALSE}
library(knitr)

default_source_hook <- knit_hooks$get('source')
default_output_hook <- knit_hooks$get('output')

knit_hooks$set(
  source = function(x, options) {
    paste0(
      "\n::: {.codebox data-latex=\"\"}\n\n",
      default_source_hook(x, options),
      "\n\n:::\n\n")
  }
)

knit_hooks$set(
  output = function(x, options) {
    paste0(
      "\n::: {.codebox data-latex=\"\"}\n\n",
      default_output_hook(x, options),
      "\n\n:::\n\n")
  }
)

knitr::opts_chunk$set(echo = TRUE)
```


```{r}
a <- c(1,2,3,4,5)
b <- c(1,2,3,4,5)
df <- data.frame(a, b)

# take a look at our data frame
df
```

---