M1.9: Create new Admin theme which will fallback to already created custom admin theme

Question

I already have custom Admin theme app/design/adminhtml/default/mycustom created and in use. But now, for some stores, I want to use another Admin theme, which will fallback to mycustom theme. I already tried defining parent theme in etc/theme.xml, but it seems like it's not working:

<theme>
    <parent>default/mycustom</parent>
</theme>

This is my theme.xml file:

<?xml version="1.0"?>
<!--
/**
 * Magento
 *
 * NOTICE OF LICENSE
 *
 * This source file is subject to the Academic Free License (AFL 3.0)
 * that is bundled with this package in the file LICENSE_AFL.txt.
 * It is also available through the world-wide-web at this URL:
 * http://opensource.org/licenses/afl-3.0.php
 * If you did not receive a copy of the license and are unable to
 * obtain it through the world-wide-web, please send an email
 * to license@magento.com so we can send you a copy immediately.
 *
 * DISCLAIMER
 *
 * Do not edit or add to this file if you wish to upgrade Magento to newer
 * versions in the future. If you wish to customize Magento for your
 * needs please refer to http://www.magento.com for more information.
 *
 * @category    design
 * @package     default_default
 * @copyright   Copyright (c) 2006-2016 X.commerce, Inc. and affiliates (http://www.magento.com)
 * @license     http://opensource.org/licenses/afl-3.0.php  Academic Free License (AFL 3.0)
 */
-->
<theme>
    <parent>default/custom_backend</parent>
</theme>

Is it possible? I know that in Magento 2 it is, but I need it for Magento 1.9.

Answer 1

You can enable custom admin theme by mention new theme path in app/etc/local.xml or in app/etc/config.xml with the following:

<config>
    <stores>
        <admin>
            <design>
                <theme>
                    <default>custom_theme</default>
                </theme>
            </design>
        </admin>
    </stores>
</config>

For enable the fallback of new admin theme with default one. Add the etc/theme.xml file in custom theme with below code:

<theme>
    <parent>default/default</parent>
</theme>