In order to install a new theme for your Ghost blog, you simply need to drop it in the folder
/var/www/ghost/content/themes/ As an example, let’s install the Coder theme.
Now that we’ve unzipped the theme, we can remove the archive and make sure the permissions are correct:
chown -R ghost:ghost Coder-master
Finally, we need to restart Ghost so that it picks up the new theme:
sudo service ghost restart
You can now select the theme on the General Setting page: