ThemeAutoloader

autoloader.php

<?php
spl_autoload_register( 'my_theme_autoloader' );
function my_theme_autoloader( $class_name ) {
	if ( substr( $class_name, 0, 6 ) == 'theme_' ) {
		$cur_theme_file    = get_stylesheet_directory() . DIRECTORY_SEPARATOR . 'inc' . DIRECTORY_SEPARATOR . $class_name . '.php';
		$parent_theme_file = get_template_directory() . DIRECTORY_SEPARATOR . 'inc' . DIRECTORY_SEPARATOR . $class_name . '.php';

		if ( file_exists( $cur_theme_file ) ) {
			require_once $cur_theme_file;
		} elseif ( file_exists( $parent_theme_file ) ) {
			require_once $parent_theme_file;
		}
	}
}