How to make rmarkdown html_document to adjust page width based on browser window size?

I am learning R Markdown to log my R code. And found the format of the R Markdown document below presented nicely:

http://rmarkdown.rstudio.com/html_document_format.html

I want to do similar thing that the page width can be adjusted automatically based on the window size of the browser.

The following is my YAML setting:

---
title: "testing"
output: 
  rmarkdown::html_document:
    toc: true
    toc_depth: 4
    toc_float:
      collapsed: false
    theme: yeti
    number_sections: true
    df_print: paged
---

How should I modify it to give the same effect as the link above? Thanks!