Modify location of main stylesheet

modify-stylesheet-location.php

<?php

/**
 * Modify location of main stylesheet
 * N.B.: style.css in the theme's root is still needed to provide WordPress
 * with the necessary metadata about the theme
 * @author Alain Schlesser (alain.schlesser@gmail.com)
 *
 * @param  string $stylesheet_uri       Stylesheet URI for the current theme/
 *                                      child theme
 * @param  string $stylesheet_dir_uri   Stylesheet directory URI for the current
 *                                      theme/child theme
 * @return string                       modified Stylesheet URI
 *
 * @see https://codex.wordpress.org/Function_Reference/get_stylesheet_uri
 * @see http://wpseek.com/hook/stylesheet_uri/
 */
function as_stylesheet_uri( $stylesheet_uri, $stylesheet_dir_uri ) {
	return $stylesheet_dir_uri . '/assets/styles/theme.css';
}
add_filter( 'stylesheet_uri', 'as_stylesheet_uri', 10, 2 );