The web part look and layout can be completely customized using html templates.
They use the same template mechanisms and follow the same rules as the ASP.NET Membership controls which they are based off of. The details for editing the memberhip control templates and a list of required fields are here:
http://msdn.microsoft.com/en-us/library/ms178339.aspx
In addition to these rules, FirstName, LastName and Email are required for the Create User Step template.
The default templates are located in:
/_layouts/##/FBA/WEBPARTS (C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\##\TEMPLATE\LAYOUTS\FBA\WEBPARTS)
(## = SharePoint version number)
Probably the easiest way to customize the template is to simply create a copy of the default template, and then edit to your heart’s content. The required fields mentioned earlier must be present in the template, but any other fields can be removed.
Once you’ve modified the template, simply point your web part to your new template in the web part properties.