No need to turn your site off, just have it set up so that the only thing visible to anonymous is the login block. Then they can use the Register link on that block to do the register thing. Or have no login block visible, and direct potential users to yoursite.com/user.php.
But you'll find most things related to color and fonts are controlled in your theme's style.css file. So if you want to change them just on this form, you'll need to create new classes etc on that template and add them to your style sheet.