Bootstrap/site/static/docs/4.3/assets/scss/_subnav.scss

.bd-subnavbar {
  background-color: rgba(#fff, .75);
  backdrop-filter: blur(1rem);
  box-shadow: 0 .5rem 1rem rgba(0, 0, 0, .05), inset 0 -1px 0 rgba(0, 0, 0, .15);

  .booticon {
    opacity: .25;
  }

  .dropdown-menu {
    @include font-size(.875rem);
    box-shadow: 0 .5rem 1rem rgba(0, 0, 0, .05);
  }

  .dropdown-item.active {
    font-weight: 600;
    color: $gray-900;
    background: escape-svg($dropdown-active-icon) no-repeat .4rem .6rem/.75rem .75rem;
  }

  @include media-breakpoint-up(md) {
    @supports (position: sticky) {
      position: sticky;
      top: 0;
      z-index: 1071; // over everything in bootstrap
    }
  }
}

.bd-search {
  position: relative; // To contain the Algolia search

  @include media-breakpoint-down(sm) {
    width: 100%;
  }

  .form-control:focus {
    border-color: $bd-purple-bright;
    box-shadow: 0 0 0 3px rgba($bd-purple-bright, .25);
  }
}

.bd-search-docs-toggle {
  line-height: 1;
  color: $gray-900;
}