discourse/app/assets/stylesheets/common/select-box-kit/categories-admin-dropdown.scss

.select-box-kit {
  &.categories-admin-dropdown {
    .select-box-kit-select-box-kit-body {
      min-width: auto;
      width: 250px;
    }

    .select-box-kit-header .d-icon {
      justify-content: space-between;
    }
  }
}