title: Is it possible to adjust background image opacity in a xaringan slideshow?
body: <p>Is it possible to adjust the opacity of a background image when creating a presentation with xaringan and R markdown? For example, here is the Ninja template with just the slide of Karl for a background. What would I need to do to change the opacity of just this background?</p>
  <p>Edit: Just to clarify, I would like to be able to adjust the background opacity on a case by case basis, not for every background image in the slideshow.</p>
  <pre><code>---
  title: &quot;Presentation Ninja&quot;
  subtitle: &quot;⚔&lt;br/&gt;with xaringan&quot;
  author: &quot;Yihui Xie&quot;
  institute: &quot;RStudio, PBC&quot;
  date: &quot;2016/12/12 (updated: `r Sys.Date()`)&quot;
  output:
    xaringan::moon_reader:
      lib_dir: libs
      nature:
        highlightStyle: github
        highlightLines: true
        countIncrementalSlides: false
  ---
  
  
  background-image: url(`r xaringan:::karl`)
  background-position: 50% 50%
  class: right, top
  
  # You only live once!
  </code></pre>