/**
 * Use a custom stylesheet for single posts.
 *
 * @param string $stylesheet     Original stylesheet URI.
 * @param string $stylesheet_dir Stylesheet directory.
 * @return string (Maybe modified) stylesheet URI.
 */
function wpdocs_custom_stylesheet_for_single_posts( $stylesheet, $stylesheet_dir ) {

	if ( is_singular( 'post' ) ) {
		$stylesheet = $stylesheet_dir . '/custom-single.css';
	}

	return $stylesheet;
}
add_filter( 'stylesheet_uri', 'wpdocs_custom_stylesheet_for_single_posts', 10, 2 );