Give your xaringan slides some style with xaringanthemer within your slides.Rmd file without (much) CSS.

First, add css: xaringan-themer.css to your xaringan slides YAML header under xaringan::moonreader:.
Then, in the knitr setup chunk, load xaringanthemer and try one of the theme functions.
```{r setup, include = FALSE}
options(htmltools.dir.version = FALSE)
library(xaringanthemer)
mono_light(
base_color = "#1c5253",
header_font_google = google_font("Josefin Sans"),
text_font_google = google_font("Montserrat", "300", "300i"),
code_font_google = google_font("Droid Mono")
)
```
xaringanthemer is Tab friendly – use autocomplete to explore the template variables that you can adjust in each of the themes!
You can also skip the above and just create a Ninja Themed Presentation from the New R Markdown Document menu in RStudio.