I don't have much experience with Magento 2. I don't know if this behaviour is a bug or i missed a step somewhere.
I have just created a custom theme derived from Magento/Luma.
Then i have created a Widget specifying the custom theme.
I noticed the theme name is blank in the column 'Design Theme' of the list of defined widgets.
Did i miss something?
Thank you for you help.
I am running Magento CE 2.1.2