Enqueue assets in a WordPress Theme

functions.php

<?php 

function enqueue_assets() {

  // If using a child theme, auto-load the parent theme style.
  if ( is_child_theme() ) {
    wp_enqueue_style( 'parent-main', get_template_directory_uri() . '/style.css' );
  }

  // Always load active theme's style.css
  wp_enqueue_style( 'main', get_stylesheet_uri() );

}

add_action( 'wp_enqueue_scripts', 'enqueue_assets' );