How to Upload Upfront Themes?

I am so glad that Upfront Builder has finally made it possible to export themes. Amazing! So wonderful.

Yet, I am now baffled whether or not I have the ability to upload themes. Is it possible to upload them through the Upfront Builder? Is is possible to upload them via FTP?

I have searched for options online, yet I haven't come to any conclusions. Could you point me in the right direction?