autoloader.php

<?php
/**
 * Autoloader 101
 *
 * @since 1.0.1
 */
spl_autoload_register( function( $class ) {
    static $base_dir   = null;
    static $subfolders = null;

    // 1. Initialize base directory and scan subfolders once
    if ( $base_dir === null ) {
        $base_dir   = get_template_directory() . '/classes/';
        $subfolders = array_filter(
            scandir( $base_dir ),
            function( $item ) use ( $base_dir ) {
                // skip ".", ".." and keep only directories
                return $item[0] !== '.' && is_dir( $base_dir . $item );
            }
        );
    }

    // 2. Extract the base class name (without namespace)
    $parts      = explode( '\\', $class );
    $class_name = array_pop( $parts );

    // 3. Normalize: lowercase and convert underscores to dashes
    $normalized = str_replace( '_', '-', strtolower( $class_name ) );

    // 4. Loop through each subfolder and look for class-theming-*.php
    foreach ( $subfolders as $folder ) {
        $file = sprintf(
            '%1$s%2$s/class-theming-%3$s.php',
            $base_dir,
            $folder,
            $normalized
        );

        if ( file_exists( $file ) ) {
            require_once $file; // load and stop scanning
            return;
        }
    }
} );