function wp_update_custom_css_post( $css, $args = array() ) {
  $args = wp_parse_args(
    $args,
    array(
      'preprocessed' => '',
      'stylesheet'   => get_stylesheet(),
    )
  );

  $data = array(
    'css'          => $css,
    'preprocessed' => $args['preprocessed'],
  );

  
/**
 * Filters the `css` (`post_content`) and `preprocessed` (`post_content_filtered`) args
 * for a `custom_css` post being updated.
 *
 * This filter can be used by plugin that offer CSS pre-processors, to store the original
 * pre-processed CSS in `post_content_filtered` and then store processed CSS in `post_content`.
 * When used in this way, the `post_content_filtered` should be supplied as the setting value
 * instead of `post_content` via a the `customize_value_custom_css` filter, for example:
 *
 * <code>
 * add_filter( 'customize_value_custom_css', function( $value, $setting ) {
 *     $post = wp_get_custom_css_post( $setting->stylesheet );
 *     if ( $post && ! empty( $post->post_content_filtered ) ) {
 *         $css = $post->post_content_filtered;
 *     }
 *     return $css;
 * }, 10, 2 );
 * </code>
 *
 * @since 4.7.0
 * @param array $data {
 *     Custom CSS data.
 *
 *     @type string $css          CSS stored in `post_content`.
 *     @type string $preprocessed Pre-processed CSS stored in `post_content_filtered`.
 *                                Normally empty string.
 * }
 * @param array $args {
 *     The args passed into `wp_update_custom_css_post()` merged with defaults.
 *
 *     @type string $css          The original CSS passed in to be updated.
 *     @type string $preprocessed The original preprocessed CSS passed in to be updated.
 *     @type string $stylesheet   The stylesheet (theme) being updated.
 * }
 */
  $data = apply_filters( 'update_custom_css_data', $data, array_merge( $args, compact( 'css' ) ) );

  $post_data = array(
    'post_title'            => $args['stylesheet'],
    'post_name'             => sanitize_title( $args['stylesheet'] ),
    'post_type'             => 'custom_css',
    'post_status'           => 'publish',
    'post_content'          => $data['css'],
    'post_content_filtered' => $data['preprocessed'],
  );

  // Update post if it already exists, otherwise create a new one.
  $post = wp_get_custom_css_post( $args['stylesheet'] );
  if ( $post ) {
    $post_data['ID'] = $post->ID;
    $r               = wp_update_post( wp_slash( $post_data ), true );
  } else {
    $r = wp_insert_post( wp_slash( $post_data ), true );

    if ( ! is_wp_error( $r ) ) {
      if ( get_stylesheet() === $args['stylesheet'] ) {
        set_theme_mod( 'custom_css_post_id', $r );
      }

      // Trigger creation of a revision. This should be removed once #30854 is resolved.
      $revisions = wp_get_latest_revision_id_and_total_count( $r );
      if ( ! is_wp_error( $revisions ) && 0 === $revisions['count'] ) {
        wp_save_post_revision( $r );
      }
    }
  }

  if ( is_wp_error( $r ) ) {
    return $r;
  }
  return get_post( $r );
}