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 {
    color: $secondary;
  }
  .category-desc {
    margin: 6px 0 0 3px;
    font-size: 12px;
  }
}