I maintain a small French WordPress site: http://entraides-citoyennes.org/ It was made by design students which made a great work with looks, but a so-so technical work... So I had to fix the issues.
They used a iFeature 5 theme by CyberChimps, they made a .css file in a child theme. I had to make CSS fixes to get labels of the registration form (made with the WP-Members plugins) to be aligned with the input fields...
WP-Members is a nice plugin, rather flexible as it allows to add custom fields to registration.