Rocketship themes currently exist as part of the rocketship_theme_generator on drupal.org.
They are automatically generated by this module, if the Rocketship distribution is used and
composer install or
composer update is run, as part of the after update or after install scripts. There is also the option of running the generator script stand-alone.
If a Dropsolid project is used to develop a Rocketship site, extra theming changes are applied on top of the themes, courtesy of the
dropsolid_theme_dependencies module. Such changes are mainly an update of the FontAwesome iconfont to the paid subscription version, as we are not allowed to distribute this on drupal.org ourselves.
Info about how the theme generator can be used and all its options, can be found as part of its readme file.
The pages of this support section go a bit deeper into how things have been developed and are a guide into making changes or additions to this generator.