Add custom code before each top-level heading in R markdown

I have an R Markdown document in which every top level heading needs the same five lines of code before it to make the formatting nice. I want to know if I can set these five lines in a style.css or latex document (or otherwise externally to the R Markdown document) rather than repeat them every time.

An example of what my R Markdown looks like at the moment - some whitespace and a line, and a \newpage so it prints nicely:

<br>
<br>
***
<br>
<br>
\newpage
# Heading 1
content
<br>
<br>
***
<br>
<br>
\newpage
# Heading 2
content
<br>
<br>
***
<br>
<br>
\newpage
# Heading 3
content

It seems like bad practice (and it's annoying) to keep repeating those five lines. It also means I'm mixing up formatting and content, which I'd prefer to avoid.

I already use a style.css file for other aspects of formatting the HTML output. I'm wondering if I can use it to do this too? Failing that, perhaps in a latex document?

Either solution would have the obvious advantages of both cleaning up my R Markdown, as well as making it simpler to adjust the formatting if I want to. (I use the same style.css for multiple related documents).

The closest I have found is this:

https://stackoverflow.com/questions/33134416/level-4-heading-issue-in-r-markdown

but I know nothin about latex documents and HTML, and am hoping there's a css solution too.

You can patch the top-level sectioning command. For example if your document uses \section as top level sectioning, you could do something like this:

---
output: pdf_document
header-includes: 
  - \pretocmd{\section}{*** \newpage}{}{}
---
# Heading 1
content
# Heading 2
content
# Heading 3
content