JSFiddle - React, Tailwind, and code Playground

HTML

<div class="details">Foo</div>

CSS

.details {
    position: relative;
    background: #dedede;
    width: 200px;
}

/* assumes .details is position: relative */
.details::after {
    content: '';
    position: absolute;
    border-top: 3px solid #7f7f7f;
    border-right: 3px solid #7f7f7f;
    height: 12px;
    width: 12px;
    top: 50%;
    right: 12px;
    margin-top: -3px;
    -webkit-transform: rotate(45deg);
    -moz-transform: rotate(45deg);
    -o-transform: rotate(45deg);
    transform: rotate(45deg);
}