jQuery addClass example

Change class name on click in jQuery

HTML

<div class="btn-2">Операционные системы</div>

CSS

.btn-2 {
  background: red;
  color: #fff;
  display: inline-block;
  padding: 7px 13px;
  position: relative;
}
.btn-2:after {
  content: '';
  position: absolute;
  bottom: 0;
  width: 300px;
  height: 1px;
  background: red;
}