#mainNav {
  border-bottom: 1px solid transparent;
  background-color: var(--bs-body-bg);
  transition: border-color 0.2s, background-color 0.2s;
  visibility: visible;
}

#mainNav.navbar-loaded {
  border-bottom-color: transparent;
}

#mainNav.navbar-loaded.navbar-shrink {
  border-bottom-color: rgba(255,255,255,0.5);
}