Returns current stylesheet registration handle.

get_current_stylesheet_handle.php

<?php
/**
 * Returns current stylesheet handle used in wp_register_style() function or NULL if not found.
 *
 * @return string|null
 */
function get_current_stylesheet_handle() {

    $pattern = sprintf(
        '/%1$s\/style\.(min\.)?css/',
        str_replace( '/', '\/', untrailingslashit( get_stylesheet_directory_uri() ) )
    );

    /** @var \_WP_Dependency $style */
    foreach ( (array) wp_styles()->registered as $style ) {
        if ( preg_match( $pattern, $style->src ) ) {
            return $style->handle;
        }
    }

    return null;
}