At the moment, you cannot change the theme of a Ghost blog hosted on Host Ghost (http://hostghost.io).
The service is still in beta and the ability to change themes is scheduled for a future update. It's not prominently displayed on the site itself, but if you hit the button to sign up for the beta, the popup requesting your email, etc. you'll also find this in the column on the right:
Coming soon - (c. Nov 1)
• Access to FTP to manage & upload themes
They should mention it in their FAQ section.