JSFiddle - React, Tailwind, and code Playground

HTML

<div id="selector"></div>

CSS

#selector{
  width:100%;
  height:100px;
  background-color:#333;
  position: relative;
  /*transform: translate(50%);*/
  /*right: 50%;
  top: 10px;*//* use margin */
  width: 95%;
  /*z-index: 20;*/
  margin: 0 auto;
}

#selector::after{
    content: '';
    width: 95%;
    height: 10px;
    position: absolute;
    right: 50%;
    bottom: -4px;
    border-radius: 5px;
    transform: translate(50%);
    background: #3D74FE;
    z-index: -1;
}