https://jhildenbiddle.github.io/css-vars-ponyfill/ with loadJS

css-custom-properties-ponyfill.html

<script>
  /*! loadJS: load a JS file asynchronously. [c]2014 @scottjehl, Filament Group, Inc. (Based on http://goo.gl/REQGQ by Paul Irish). Licensed MIT */
  !function(e){var t=function(t,o,n){"use strict";var r,a=e.document.getElementsByTagName("script")[0],c=e.document.createElement("script");return"boolean"==typeof o&&(r=n,n=o,o=r),c.src=t,c.async=!n,a.parentNode.insertBefore(c,a),o&&"function"==typeof o&&(c.onload=o),c};"undefined"!=typeof module?module.exports=t:e.loadJS=t}("undefined"!=typeof global?global:this);

  // Polyfill custom properties for <= IE11
  if (!window.CSS || !window.CSS.supports || !window.CSS.supports('--a', 0)) {
    loadJS('https://unpkg.com/css-vars-ponyfill@2/dist/css-vars-ponyfill.min.js', function() {
      cssVars();
    })
  }
</script>

Safari gives false positives here, so if you're just looking to polyfill IE, add documentMode:

(!window.CSS || !window.CSS.supports || !window.CSS.supports('--a', 0)) && document.documentMode