Themes are (usually) only styling front-end of your site. Drupal by default (must) have users system login/registration form and all other functionality related to users.
When you activate your new theme logout and go to page “/user” to see your loging form. There are also “/user/register”, “/user/password” and some other related with user account functionality.
Will your custom theme also style login/registration/pass forgot forms (pages) or not – that’s another question. But they will definitely exist. But even if they are not styled you can put on your own some extra CSS to style them.
For Drupal 7 file field is already included in basic installation (for D6 you have to install extra module for that: https://www.drupal.org/project/filefield ).
solved can someone pls help me to find out opensource drupal themes which has user login options and file upload