Zend certified PHP/Magento developer

Changing phtml file via /app

In a previous question i asked where to find items.phtml file.

I found it and did the configuration and it worked. But when i did this it was via this path: vendor/magento/module-shipping/view/adminhtml/templates/create/items.phtml.

I read somewhere that they do not recommend to any changes via /vendor but do this via /app/design/adminhtml/custom_vendor/custom_theme/ and so on.

What i did was i downloaded the items.phtml (un-fonfigured) and i did the change and tried to add this file to my created path:
/app/design/adminhtml/custom_vendor/custom_theme/templates/create/items.phtml

after doing the setup:upgrade and setup:deploy, cleared cache and re-indexed nothing happen.

Can anyone help and tell me what i have done wrong?

Any tips and help is appreciated.

Thanks