Adding your theme to Drupal.org
Theme contributors are strongly encouraged to host your themes at Drupal.org rather than elsewhere (GitHub, BitBucket, your personal site, etc.). To add your theme to Drupal.org, it must carry the GPL license. This implies that copyrighted works or images that you do not want to see re-used or otherwise altered, should not be included in your theme.
Themes are tracked the same way that code is, namely in the git repository. You will need to apply for a git account and your theme will be reviewed by reviewers. You should make sure your theme complies with Drupal coding standards. Once your account application is approved, you will be able to check your theme into the Drupal git repository. Once you create a project, the download capability will be created for it automatically.
If you do add your theme, then over time, users will likely post suggestions, file bugs, and generally encourage that you keep the theme up to date with current versions of Drupal.
Also read the screenshot guidelines.
Before submitting a theme, check that it meets the standards for accessibility. There are a number of tools and other resources that can help you evaluate the accessibility of your theme.
You can read more details about maintaining a project on Drupal.org in the Getting Involved book.
Often, theme designers prefer graphical user interfaces (GUIs) for git, rather than command line. For information on using them with the Drupal repositories, see Git clients and GUI tools.