@media (min-width:992px) {
  #mainNav {
    padding-top: 20px;
    padding-bottom: 20px;
    transition: background .3s ease-in-out,padding-top .3s ease-in-out,padding-bottom .3s;
    letter-spacing: 1px;
    border-bottom: none;
    background: rgba(0, 0, 0, 0.5);
  }
}