you need to add a custom module and define css in desired xml or if you want to add in all pages create default.xml
1) add a module Ex. Emizentech/NewAdmin

2) create a new file view/adminhtml/layout/default.xml in the module

3) Add the .css file in web folder as below path:

after this, you need to clear cache and deploy static content

Magento 2.0 override admin css