JSFiddle - React, Tailwind, and code Playground

HTML

<div style="background-color: red;" class="block-container">
    Hello World
</div>

<div style="background-color: red;" class="block-container">
    Hello World 2
</div>

JavaScript

var blocks = document.getElementsByClassName("block-container");
for (var i = 0; i < blocks.length; i++) {
    var block = blocks[i];
    block.style.backgroundColor = "#252525";
}