It seems you forgot to mention that you also need a bit of code that goes into lib-common.php to makt this work:
if (!empty($usetheme) AND is_dir($_CONF['path_themes'] . $usetheme)) {
$_CONF['theme'] = $usetheme;
$_CONF['path_layout'] = $_CONF['path_themes'] . $_CONF['theme'] . '/';
$_CONF['layout_url'] = $_CONF['site_url'] . '/layout/' . $_CONF['theme'];
}
else if ($_CONF['allow_user_themes'] == 1) {
Only that last line (minus the "else") is already there ... Add this and then it will work.
bye, Dirk