The way I suggest you do this is to create a specific login page. You can selecte the *hidden* option when making the page so it's not displayed anywhere. Then set your content in the page.
Here's an example of what how I've done what I think you are trying to achieve.
http://www.schoolofgann.com/ this is the home page for the site. But I want to manage how the login page looks, so, if you click the Login button it automatically goes to this page - http://www.schoolofgann.com/site+login.aspx.
To do this, I create the page first AND PUT THE LOGIN MODULE IN THERE FIRST
Then, go to the admin/ SiteSettings page, and scroll down to advanced, and you can choose the LOGIN page. If you set that login page in there, DotNetNuke automatically tells the Login link which is in the skin to go to the Login Page you have created.
You can also set this for the registration page.
The same thing happens on my XD site - http://www.xd.com.au but in a different way. I've set the Login Page to be the home page. BUT I have set the Login Module to be visible ONLY to unauthenticated people - that basically means - anyone who is not logged in. Once you log in, (except as administrator of course) it goes away.
So you can see here - there are multiple ways to use the login function and you can understand now the skinning and design works together with the actual functionality of DotNetNuke.
I hope this helps you.
Nina Meiers