7.8 CSS and themes

The format xaringan::moon_reader has a css option, to which you can pass a vector of CSS file paths, e.g.,

---
output:
  xaringan::moon_reader:
    css:["default", "extra.css"]
---

The file path should contain the extension .css. If a path does not contain a filename extension, it is assumed to be a built-in CSS file in the xaringan package.
Make sure to insert the relative path to the file if it is not placed in the same directory as your .Rmd file.
When you specify multiple CSS files, the one that comes later will take priority and override the previous ones if there are conflicting CSS rules.

To see all built-in CSS files, call xaringan:::list_css() in R.

xaringan:::list_css() # print path of built-in CSS files
names(xaringan:::list_css()) # print names only

When you only want to override a few CSS rules in the default theme, you do not have to copy the whole file default.css; instead, create a new (and hopefully smaller) CSS file that only provides new CSS rules.
Users have contributed a few themes to xaringan. For example, you can use the metropolis theme (https://github.com/pat-s/xaringan-metropolis):