JSFiddle - React, Tailwind, and code Playground

HTML

<div>нажми</div>

CSS

@import url('https://fonts.googleapis.com/css2?family=Neucha&display=swap');

:root {
  --color: red;
}

* {
  box-sizing: inherit;
}

html {
  box-sizing: border-box;
}

body {
  margin: 0;
  font-family: 'Neucha', cursive;
}

div {
  display: flex;
  align-items: center;
  justify-content: center;
  flex-wrap: wrap;
  width: 100px;
  height: 100px;
  background: #eee;
  text-align: center;
}
div::after {
  content: "";
  display: block;
  width: 100%;
  height: 30px;
  background: var(--color);
}

JavaScript

const block = document.querySelector('div');
block.addEventListener('click', () => {
  block.style.setProperty('--color', 'blue');

})