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/

What if it’s a paid theme @asb? This won’t work for that because the folder is on my desktop.