Add and load a new style. Version is used to force reload the style and not using cached style, because "style.css?ver=1.0" is different than "style.css?ver=2.0".
<?php // Inside a parent theme wp_enqueue_style( 'my-style', get_template_directory_uri() . '/css/my-style.css', false, '1.0', 'all' ); // Inside a child theme wp_enqueue_style( 'my-style', get_stylesheet_directory_uri() . '/css/my-style.css', false, '1.0', 'all' ); // Inside a plugin wp_enqueue_style( 'my-style', plugins_url( '/css/my-style.css', __FILE__ ), false, '1.0', 'all' ); ?>