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 {
    /* leave this color as non-variable -- it is displayed on a white background in a dropdown */
    color: #333;
    margin: 6px 0 0 3px;
    font-size: 0.857em;
    line-height: 16px;
  }
}