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.

cd /var/www/ghost/content/themes/
https://github.com/dongri/Coder/archive/master.zip
unzip master.zip

Now that we've unzipped the theme, we can remove the archive and make sure the permissions are correct:

rm master.zip
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: http://your-blog.com/ghost/settings/general/