Overriding Magento backend theme isn't working

[jabrware]

After you moved content of app/design/adminhtml/Magento/backend theme to vendor/theme-adminhtml-backend, putting any overriding files in app/design/adminhtml/Magento/backend isn't processed at all
and isn't deployed to pub/static/...

Why?
And how to override backend theme without touching the core files?

[davidalger]

The GitHub issue tracker is intended for technical issues only. Please refer to the Community Forums or Magento Stack Exchange site for technical questions. In your case, the programming questions forum is likely the most appropriate. Feel free to reopen this issue if you think you have encountered a bug in Magento 2.