JSFiddle - React, Tailwind, and code Playground

HTML

<div class="block"></div>
<div class="block m-rounded"></div>
<div class="block m-rounded m-dark"></div>

CSS

.block {
    width: 100px;
    height: 100px;
    
    display: inline-block;
    margin: 10px;
    
    border: 10px solid #aaa;   
}

.block.m-rounded {
    border-radius: 50%;
}

.block.m-dark {
    background: #333;
}