JSFiddle - React, Tailwind, and code Playground

HTML

<div class="css-variable-test"></div>

CSS

:root {
  --main-bg-color: rgb(1, 2, 3);
}
    
.css-variable-test {
    display: none;
    background-color: var(--main-bg-color);
}

JavaScript

var computedStyle = getComputedStyle(document.getElementsByClassName('css-variable-test')[0], null);

if (computedStyle.backgroundColor == "rgb(1, 2, 3)") {
    alert('CSS variables support');
}