There seem to be some plugins that allow you to modify the look and feel of the WordPress Registration Page. But what if I want to extend my theme to it. Is this possible?

Perhaps I have to more or less design the page from scratch using my theme as a guide to how the page works. I have noticed that some of the plugins require that I supply my own css and php file. How sould this these files be designed?

I need a modification to the create account script that will initially show only two text fields.. firstname and lastname. The new user will enter these and click 'continue', which will start the usual registration form, and fill in the username automatically with 'firstname.lastname' -- it would be best if the username field was set to readonly, so the new user couldn't change the generated username.