title: Changing the font size of figure captions in RMarkdown HTML output
body: <p>I would like to make the font size of all figure captions in my R Markdown document smaller. The final output is HTML and I'm working in R Studio. To load the picture, I use the <code>include_graphics</code> function from <code>knitr</code>, because I've been told it's the best way (see <a href="http://www.zevross.com/blog/2017/06/19/tips-and-tricks-for-working-with-images-and-figures-in-r-markdown-documents/#default-settings-for-including-images-and-figures-in-r-markdown" rel="noreferrer">here</a>). My .Rmd file is:</p>
  
  <pre><code>---
  title: "ppp"
  author: "ppp"
  date: "July 4, 2017"
  output: 
    html_document: 
      fig_caption: yes
  ---
  
  ```{r setup, include=FALSE}
  library(knitr)
  opts_chunk$set(echo = FALSE)
  ```
  
  
  ```{r foo, fig.cap="$f_{p}$ as a function of $g$ for various values of $r=\\frac{\\rho_{w}}{\\rho_{a}}$"}
  # All defaults
  include_graphics("download.jpg")
  ``` 
  
  This is regular text.
  </code></pre>
  
  <p>The corresponding output is:
  <a href="https://i.stack.imgur.com/qchms.png" rel="noreferrer"><img src="https://i.stack.imgur.com/qchms.png" alt="enter image description here"></a></p>
  
  <p>As you can see, the caption font size and the regular text font size are <em>exactly</em> the same, which doesn't look that nice. How can I solve this problem?</p>