Add the CSS to your child themes style.css file or load it using wp_enqueue_scripts
add_action( 'wp_enqueue_scripts', 'wpsites_load_custom_styles' );
function wpsites_load_custom_styles() {
wp_register_style( 'custom-css', get_stylesheet_directory_uri() . '/your-style.css', false, '1.0.0' );
wp_enqueue_style( 'custom-css' );
}