Yup, that will work just fine. We even have some extra code to allow overriding our stylesheet so you can copy either, or both, and override just what you need.
So, you can copy the default.php and the loginregister.css file from the /components/com_jfbconnect/views/loginregister/tmpl/ directory to /templates/<YOUR_TEMPLATE>/html/com_jfbconnect/loginregister/ and then edit either however you want.
Hope that helps, but if you run into any issues, just let us know.
Alex
Proper way to override template
