Collapse.css

.collapse {
  display: none;
}

.collapse.open {
  display: block;
}

.navbar-collapse  {
  -ms-flex-preferred-size: 100%;
  flex-basis: 100%;
  -webkit-box-flex: 1;
  -ms-flex-positive: 1;
  flex-grow: 1;
  -webkit-box-align: center;
  -ms-flex-align: center;
  align-items: center;
}

.navbar-collapse.open {
  padding-bottom: 1rem;
}

.navbar-collapse.open .navbar-nav .nav-link,
.navbar-collapse.open .navbar-nav .dropdown-toggle {
  line-height: 35px;
  padding: .5rem 0;
}