Projectfork has its own installer for extensions and themes, so you don't use the normal Joomla! installer (you'll get an error or white page if you attempt to install a Projectfork theme in the Joomla! installer).

1. Go to Projectfork > Config > Themes
2. Browse for your compressed (usually zip) theme file
3. Click Install
4. Publish the newly installed theme
5. Click the star to set the theme as the Default theme