title: Change the color of header in Rmarkdown Prettydoc templates
body: <p>Using the package <code>prettydoc</code> in R Markdown, there is a template, the code is below</p>
  <pre><code>---
  title: Nineteen Years Later
  author: Harry Potter
  date: July 31, 2016
  output:
    prettydoc::html_pretty:
      theme: cayman
      highlight: github
  ---
  </code></pre>
  <p>The output look likes this</p>
  <p><a href="https://i.stack.imgur.com/LC7Z1.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/LC7Z1.png" alt="enter image description here" /></a></p>
  <p>Was wondering where I would be able to change the colors that transcend, so maybe it could go from a red to orange or anything I declare. Where is the .css for this header located? Thank you in advance</p>