You cant customise login page using admin section , you have to hard code it manually, its up to you what changed you want to make , for design edit css for login page , for functionality modify the common controller .
But i want to customize the page only for admin. and for users it should remain the same
The Login page is same for both the users and administrators, if you want to add a different page for admin login , you need to create a separate view which is out of the box ,
