discourse/app/assets/stylesheets/common/select-kit/single-select.scss

.select-kit.single-select {
  .select-kit-filter {
    display: flex;

    &.is-expanded {
      padding: 0.25em 0.5em;
      border-top: 1px solid $primary-low;
      border-bottom: 1px solid $primary-low;
    }
  }
}