title: How to feature detect for CSS custom properties?
body: <p>Is there any way to detect whether a browser supports <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/--*" rel="noreferrer">CSS custom properties</a> (eg. <code>color: var(--primary)</code>)?</p>
  
  <p>I need to write a script that behaves slightly differently in browsers that don't support custom properties, but I can't find any documented <a href="https://modernizr.com/" rel="noreferrer">Modernizr</a> tests, nor any information about the Javascript interface for accessing custom properties.</p>
  
  <p>Would be grateful for any suggestions!</p>