JSFiddle - React, Tailwind, and code Playground

HTML

<div class="a1">This is a test.</div>
<div class="a1">This is also a test.</div>
<div class="a1">This is the last test.</div>
<button onclick="changeColor();">Change color to white</button>
<script>
function changeColor() {
    for (var i = 0; i < document.getElementsByClassName("a1").length; i++) {
        document.getElementsByClassName("a1")[i].style.color = "#FFFFFF";
    }
}
</script>script>

CSS

.a1 {
    background-color: black;
}

JavaScript

function changeColor() {
    for (var i = 0; i < document.getElementsByClassName("a1").length; i++) {
        document.getElementsByClassName("a1")[i].style.color = "#FFFFFF";
    }
}