Change font size of ATX-header in markdown - formatting

Question

I am writing a book with bookdown. Unfortunately, I have no clue how to format (e.g. setting font size) ATX-hearder (#, ##, ## etc.). So far, it does not work via pandoc or preamble.tex.
I have tried the following, with regard to this.
Unfortunately, there is an error message :
\usepackage{titlesec} \titleformat{\chapter}[display] {\normalfont\sffamily\huge\bfseries\color{blue}} {\chaptertitlename\ \thechapter}{20pt}{\Huge}
Thanks in advance!

Answer 1

Your best bet here is to add a LaTeX preamble to the document. In here, you can define the required LaTeX packages. Two changes are made to the base template:
We need to add subparagraph: true to make titlesec work with R Markdown, as explained here
# refers to a level one header in pandoc, and therefore you need to make the style changes for section not chapter https://www.sharelatex.com/learn/Sections_and_chapters
Here is a minimal example
---
output:
pdf_document:
includes:
in_header: header.tex
subparagraph: true
---
# Section
## Subsection
The preamble.tex file is saved in the same directory:
\usepackage{titlesec}
\usepackage{color}
\titleformat*{\section}{\LARGE}
\titleformat{\subsection}[display]
{\normalfont\sffamily\huge\bfseries\color{blue}} {\chaptertitlename\ \thechapter}{20pt}{\Huge}