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

//
// Main navbar
//

.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-toggle {
    @include font-size(100%);
    text-decoration: none;
  }

  .dropdown-menu {
    @include font-size(90%);
    box-shadow: 0 .5rem 1rem rgba(0, 0, 0, .05);
  }

  @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;
}

.link-dark {
  font-weight: 500;
  color: $dark;

  &:hover,
  &:focus {
    color: $blue;
    text-decoration: none;
  }
}

.bd-navbar {
  min-height: 4rem;
  background-color: $bd-purple-bright;

  @include media-breakpoint-down(md) {
    .navbar-nav-scroll {
      width: 100%;
      height: 2.5rem;
      margin-top: .25rem;
      overflow: hidden;

      .navbar-nav {
        padding-bottom: 2rem;
        overflow-x: auto;
        white-space: nowrap;
        -webkit-overflow-scrolling: touch;
      }
    }
  }

  .navbar-nav {
    .nav-link {
      padding-right: .5rem;
      padding-left: .5rem;
      color: $bd-purple-light;

      &.active,
      &:hover,
      &:focus {
        color: $white;
        background-color: transparent;
      }

      &.active {
        font-weight: 600;
      }
    }
  }

  .navbar-nav-svg {
    display: inline-block;
    width: 1rem;
    height: 1rem;
    vertical-align: text-top;
  }

  .dropdown-menu {
    @include font-size(.875rem);
  }

  .dropdown-item.active {
    font-weight: 600;
    color: $gray-900;
    background-color: transparent;
    background-image: url("data:image/svg+xml,%3csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 8 8'%3e%3cpath fill='%23292b2c' d='M2.3 6.73L.6 4.53c-.4-1.04.46-1.4 1.1-.8l1.1 1.4 3.4-3.8c.6-.63 1.6-.27 1.2.7l-4 4.6c-.43.5-.8.4-1.1.1z'/%3e%3c/svg%3e");
    background-repeat: no-repeat;
    background-position: .4rem .6rem;
    background-size: .75rem .75rem;
  }
}