discourse/app/assets/stylesheets/common/base/combobox.scss

.category-combobox, .select2-drop {
  .badge-category {
    display: inline-block;
  }
  .topic-count {
    font-size: 11px;
    color: $primary;
    display: inline-block;
  }
  .highlighted .topic-count, .select2-highlighted .category-desc {
    color: $primary;
  }
  .category-desc {
    color: scale-color($primary, $lightness: 45%);
    margin: 6px 0 0 3px;
    font-size: 12px;
    line-height: 16px;
  }
}