We are currently looking to customize a customer's single sign on page.  They would like to do thinks like add a logo, change colors and insert the domain name automatically for users' logins.  

I have spent some time and have seen that playing with the login.cshtml and loginform.min.css in the \Directory Server\Web\WebSTS\Content\LoginForms allows us to change the image and add some text to the page. 

Would we also use the login.cshtml page t o write a script to insert the domain name for the user authentication?  The idea is that users would only have to enter their username and that entering the domain would not be necessary.