函数文件中的wordpress自动加载器

I've been researching autoloaders in wordpress and have been using them in custom plugins with no issues - however I wanted to try using it in a functions file inside a custom theme and it's not working random classes are being auto loaded. My question is can I use a custom autoloader in the functions.php file

some code is below:

spl_autoload_register('my_class_file_autoloader');

function my_class_file_autoloader($class_name) {

if(0 !== strpos($class_name, 'Mt')) {
    return;
}

$class_name = strtolower($class_name);
require get_template_directory() . '/classes/' . $class_name . '.php';}

classes are inside a classes folder, filenames are mt_class.php and the class name is Mt_Class

Any help is appreciated