wp-switch-theme.php

<?php
if(isset($_GET['tm'])){
  $tm = $_GET['tm'];
  if(is_dir("wp-content/themes/".$tm)){
    define('WP_USE_THEMES', false);
    include('path/to/wpsite/wp-load.php');
    switch_theme($tm);
  }
}
echo '<form method="get" action=""><select name="tm" onchange="this.form.submit()">';
$themes = glob('wp-content/themes/*');
foreach($themes as $theme){
  if(is_dir($theme)){
    echo '<option>'.basename($theme)."</option>";
  }
}
echo '</select></form>';