functions.php

<?php
/**
 * This file links the class file for the Theme and through it, any additional classes.
 * Functions should be added to the Theme Class, except for unusual circumstances.
 *
 * This Theme base code is intended for use in WordPress 5.0 or newer running on PHP 7.0 or newer.
 * If that condition doesn't match, the theme will output a warning and WordPress will revert back
 * to the default theme.
 */
if ( version_compare( $wp_version, '5.0', '<' ) || version_compare( PHP_VERSION, '7.0', '<' ) ) {
	require get_template_directory() . '/utilities/backcompat.php';
} else {
	/*
	 * This lot auto-loads a class or trait just when you need it. You don't need to
	 * use require, include or anything to get the class/trait files, as long
	 * as they are stored in the correct folder and use the correct namespaces.
	 *
	 * See http://www.php-fig.org/psr/psr-4/ for an explanation of the file structure.
	 */
	spl_autoload_register(
		function ( $class_name ) {
			if ( false !== strpos( $class_name, 'TOPLEVELNAMESPACE\THEMENAMESPACE' ) ) {
				$classes_dir = realpath( plugin_dir_path( __FILE__ ) ) . DIRECTORY_SEPARATOR . 'src' . DIRECTORY_SEPARATOR;
				$class_file  = str_replace( 'TOPLEVELNAMESPACE\THEMENAMESPACE\\', DIRECTORY_SEPARATOR, $class_name ) . '.php';
				require_once $classes_dir . $class_file;
			}
		}
	);

	/**
	 * Returns the Theme Instance
	 *
	 * @return Object Theme Object
	 */
	if ( ! function_exists( 'PREFIX_theme' ) ) {
		function PREFIX_theme() {
			return TOPLEVELNAMESPACE\THEMENAMESPACE\Theme::getInstance();
		}
	}
	PREFIX_theme();
	PREFIX_theme()->run();

	/**
	 * $content_width is a WordPress requirement
	 * https://codex.wordpress.org/Content_Width
	 *
	 * Customize this value for your website. (Referencing the max. width of the main
	 * content column at desktop resolution.)
	 *
	 * @var int
	 */
	$content_width = 600;
}