header {
  nav {
    @extend .navbar;
    @extend .navbar-expand-lg;
    @extend .navbar-light;
    @extend .bg-light;
    div.topbar-container {
      @extend .container;
      >a {
        @extend .navbar-brand;
      }
      button.expand {
        @extend .navbar-toggler;
        i {
          @include icon(bars);
        }
      }
      .nav-container {
        @extend .collapse;
        @extend .navbar-collapse;
        .main-bar {
          @extend .navbar-nav;
          @extend .mr-auto;
          li {
            @extend .nav-item;
            a {
              @extend .nav-link;
            }
          }
        }
        .account-bar {
          @extend .navbar-nav;
          li {
            @extend .nav-item;
            a {
              @extend .nav-link;
            }
          }
          .user {
            @extend .navbar-text;
            padding-left: 0.5rem;
            padding-right: 0.5rem;
            i {
              @include icon(user);
            }
          }
          .settings {
            i {
              @include icon(cog);
            }
          }
          .logout {
            i {
              @include icon(power-off);
            }
          }
          .login {
            i {
              @include icon(power-off);
            }
          }
        }
      }
    }
  }
}

header {
  nav {
    @extend .navbar;
    @extend .navbar-expand-lg;
    @extend .navbar-light;
    @extend .bg-light;
    div.topbar-container {
      @extend .container;
      >a {
        @extend .navbar-brand;
      }
      button.expand {
        @extend .navbar-toggler;
        i {
          @include icon(bars);
        }
      }
      .nav-container {
        @extend .collapse;
        @extend .navbar-collapse;
        .main-bar {
          @extend .navbar-nav;
          @extend .mr-auto;
          li {
            @extend .nav-item;
            &.dropdown {
              @extend .dropdown;
              .dropdown-menu {
                @extend .dropdown-menu;
                a {
                  @extend .dropdown-item;
                }
              }
            }
            a {
              @extend .nav-link;
            }
          }
        }
        .account-bar {
          @extend .navbar-nav;
          li {
            @extend .nav-item;
            a {
              @extend .nav-link;
            }
          }
          .user {
            @extend .navbar-text;
            padding-left: 0.5rem;
            padding-right: 0.5rem;
            i {
              @include icon(user);
            }
          }
          .settings {
            i {
              @include icon(cog);
            }
          }
          .logout {
            i {
              @include icon(power-off);
            }
          }
          .login {
            i {
              @include icon(power-off);
            }
          }
        }
      }
    }
  }
}

Add some style for dropdown menus in nav
Add some style for dropdown menus in nav

Backport to starter projects
