Source

File: wp-includes/theme.php

function wp_get_custom_css_post( $stylesheet = '' ) {
	if ( empty( $stylesheet ) ) {
		$stylesheet = get_stylesheet();
	}

	$custom_css_query_vars = array(
		'post_type'              => 'custom_css',
		'post_status'            => get_post_stati(),
		'name'                   => sanitize_title( $stylesheet ),
		'posts_per_page'         => 1,
		'no_found_rows'          => true,
		'cache_results'          => true,
		'update_post_meta_cache' => false,
		'update_post_term_cache' => false,
		'lazy_load_term_meta'    => false,
	);

	$post = null;
	if ( get_stylesheet() === $stylesheet ) {
		$post_id = get_theme_mod( 'custom_css_post_id' );

		if ( $post_id > 0 && get_post( $post_id ) ) {
			$post = get_post( $post_id );
		}

		// `-1` indicates no post exists; no query necessary.
		if ( ! $post && -1 !== $post_id ) {
			$query = new WP_Query( $custom_css_query_vars );
			$post  = $query->post;
			/*
			 * Cache the lookup. See wp_update_custom_css_post().
			 * @todo This should get cleared if a custom_css post is added/removed.
			 */
			set_theme_mod( 'custom_css_post_id', $post ? $post->ID : -1 );
		}
	} else {
		$query = new WP_Query( $custom_css_query_vars );
		$post  = $query->post;
	}

	return $post;
}