I think retaining the default style from login.html / login.css can be a hassle, not sure how to go about that when trying to design the new page in Mendix. You could look at the structure and try to rebuild it with grids and containers and use the same classes. Then rewrite your login.css to a _login.scss (SASS) file and have it compiled with Calypso so the styling is incorperated in your main css.
Be careful when allowing anonymous users to your project. Set up your authorization in a separate module and only give your anonymous users access to that module to prevent security issues.