twbs--bootstrap/site/static/docs/4.3/assets/scss/_sidebar.scss

// stylelint-disable declaration-no-important

.bd-links {
  @include media-breakpoint-up(md) {
    @supports (position: sticky) {
      position: sticky;
      top: 5rem;
      height: calc(100vh - 7rem);
      overflow-y: auto;
    }
  }

  // Override collapse behaviors
  @include media-breakpoint-up(md) {
    display: block !important;
  }
}

:not(.active) > .bd-sidenav {
  display: none;
}

.bd-sidenav-group-link {
  padding: .25rem .625rem .25rem .5rem;
  font-weight: 600;
  color: rgba($black, .65);
  @include border-radius(.25rem);

  > * { pointer-events: none; }

  &:hover,
  &:focus {
    color: rgba($black, .85);
    text-decoration: none;
    background-color: rgba($bd-purple-bright, .1);
  }
}

.bd-sidenav-group {
  &.has-children .bd-sidenav-group-link::before {
    display: inline-block;
    margin-right: .25rem;
    line-height: 0; // Align in the middle
    content: escape-svg($sidebar-colapse-icon);
  }

  &.active {
    .bd-sidenav-group-link::before {
      transform: rotate(90deg);
    }

    > .bd-sidenav-group-link {
      color: rgba($black, .85);
    }
  }
}

// All levels of nav
.bd-sidebar .nav {
  padding-left: 1.25rem;

  > li > a {
    display: inline-block;
    padding: .25rem .5rem;
    @include font-size(.875rem);
    color: rgba($black, .65);
    @include border-radius(.25rem);

    &:hover,
    &:focus {
      color: rgba($black, .85);
      text-decoration: none;
      background-color: rgba($bd-purple-bright, .1);
    }
  }

  > .active > a,
  > .active:hover > a,
  > .active:focus > a {
    font-weight: 600;
    color: rgba($black, .85);
  }
}