using different font for headers/title vs body of rmarkdown pdf

Question

I'm writing an automated PDF report (Word doc is not an option) for a state agency in Rmarkdown, and need to adhere to their font requirements. They require Fira Sans for the main text, and Anton for the title and headers. I'm able to set Fira Sans as the main font, but it is unclear to me how to change title/headers. Here's a reproducible example:

---
title: "Untitled"
mainfont: Fira Sans
output:
  pdf_document:
    latex_engine: xelatex
---

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

## R Markdown

This is an R Markdown document. Markdown is a simple formatting syntax for authoring HTML, PDF, and MS Word documents. For more details on using R Markdown see <http://rmarkdown.rstudio.com>.

Is there a way to set that option in the YAML section or document itself?

Answer 1

This worked for what I needed (along with some other formatting to control size/spacing):

---
title: "Untitled"
mainfont: Fira Sans
header-includes:
  - \usepackage{fontspec}
  - \usepackage{titlesec}
  - \usepackage{titling}
  - \pretitle{\centering\Huge\fontspec{Anton}}
  - \posttitle{\vspace{0.5 cm}\centering\Huge\fontspec{Anton}}
  - \usepackage{sectsty}
  - \sectionfont{\LARGE\fontspec{Anton}}
output: 
  pdf_document:
  latex_engine: xelatex
---