Magento 2: Admin theme installation

Question

I have followed the dev docs on 2.2 to create an admin theme.

http://devdocs.magento.com/guides/v2.2/frontend-dev-guide/themes/admin_theme_create.html

When I run php bin/magento setup:upgrade I receive the following error, but I even checked my module against the admin theme dev docs again.

Module 'Magento_Theme':

Running data recurring...Field title can't be empty

my app/design/Snow/wonderland/theme.xml file

<theme xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/theme.xsd">
    <title>Wonderland</title> <!-- your theme's name -->
    <parent>Magento/backend</parent> <!-- the parent theme. Example: Magento/backend -->
</theme>

my registration.php file

<?php
\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::THEME,
    'adminhtml/Snow/wonderland', // Example: 'adminhtml/Magento/backend'
    __DIR__
);

my /etc/adminhtml/di.xml

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">

    <!-- Admin theme. Start -->
    <type name="Magento\Theme\Model\View\Design">
        <arguments>
            <argument name="themes" xsi:type="array">
                <item name="adminhtml" xsi:type="string">Snow/wonderland</item> <!-- Example: "Magento/backend" -->
            </argument>
        </arguments>
    </type>
    <!-- Admin theme. End -->
</config>

My /etc/module.xml

<module name="Snow_wonderland" setup_version="2.0.1"> <!-- Example: "Magento_Backend -->"
    <sequence>
        <module name="Magento_Theme"/>
    </sequence>
</module>

Answer 1

You forgot to add a title value in the theme.xml file.

<theme xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/theme.xsd">
    <title>[VALUE HERE]</title>
    <parent>Magento/blank</parent>
</theme>

EDIT:

Make sure you have your theme under app/design/adminhtml/Vendor/Theme. A link a about backend themes here.