Magento2 unable to load theme error with Custom Admin theme

Question

I have created new admin theme and used in my custom module di.xml file like below:

<type name="Magento\Theme\Model\View\Design">       
        <arguments>
            <argument name="themes" xsi:type="array">
                <item name="adminhtml" xsi:type="string">Newtheme/backend</item>
            </argument>
        </arguments>
    </type>

but when i send any email from admin order, it throws error in system.log file like below:

Unable to load theme by specified key: '' [] []

Can anyone let me know what could be issue?i have done upgrade,compile,deploy and cache flush all the things.