Just echoing what Chip already said: Themes are for presentation, plugins are for functionality. Custom user registration and a ‘My Account’ panel would be in the realm of custom functionality.
To quote another WordPress developer:
If you’re building a bathroom and you change the wallpaper, the toilet shouldn’t disappear, too.
Themes are the look and feel of your site and should be capable of being interchanged and swapped out easily. Changing the theme, though, shouldn’t remove your custom features (i.e. user management tools). This is why they should be housed in a plugin rather than the theme.
That said, Chip also recommended an outstanding system: Members Plugin.
If that’s not quite what you’re looking for, check out these as well:
Finally, since the custom work you need done might fall outside the realm of any existing plugin, you might consider listing the project on jobs.wordpress.net and hiring a developer to build the system for you.