on your page, the css for the checkout form will be set in the following file:
webassist/webassist/forms/fd_newfromblank_default.css
if you want to completely remove the styling, you can delete the reference to that file and create your own css for the checkout form, or you can edit the styles in that css.
I sugest getting familiar with using firebug for css troubleshooting:
http://www.webassist.com/tutorials/Using-Firebug
adding registration and login to the checkout system is covered in this tutorial:
http://www.webassist.com/tutorials/Applying-Register-Login-to-your-Checkout-Process