Test for CSS Variables

css-variables.js

function testCSSVariables() {
 var color = 'rgb(255, 198, 0)';
 var el = document.createElement('span');

 el.style.setProperty('--color', color);
 el.style.setProperty('background', 'var(--color)');
 document.body.appendChild(el);

 var styles = getComputedStyle(el);
 var doesSupport = styles.backgroundColor === color;
 document.body.removeChild(el);
 return doesSupport;
}

testCSSVariables();

There's also the JavaScript API CSS.supports().

var supportsVariables = CSS.supports('--css-variables', 1);
var supportsFlexbox= CSS.supports('display', 'flex');

Unfortunately, is doesn't have IE support, but you can polyfill it for now.

Cool article on CSS @supports & JavaScript CSS.supports()