Howdy. I’ve never modified the default mechanism. No that you’ve brought it up, looks like I will. Ugly. Been awhile since I faked a new user registration. One more thing on the list.
I took a look at wp-signup.php and it calls the theme’s header and footer and has it’s own internal css. We gotta modify that or go get a plugin. It doesn’t know anything about your theme’s sidebars.