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.