Specifically, I wanted to install this responsive backend theme so I could get convenient access from my phone.

https://www.odoo.com/apps/themes/10.0/backend_theme_v10/

I found the sources on git : https://github.com/Openworx/backend_theme.git

use git clone to checkout the sources into your /opt/odoo10/custom/addons folder. All themes are addons so they go in this folder. Confirm that this custom folder is part of the addons path in the config file (/etc/odoo/odoo.conf for odoo 10)

cd /opt.odoo10/custom/addons/
git clone --depth 1 https://github.com/Openworx/backend_theme.git

the problem is that git clone doesn’t do the folder structure exactly as Odoo expects it so create a symlink to the proper directory.

ln -s /opt/odoo10/custom/addons/backend_theme/backend_theme_v10 /opt/odoo10/custom/addons/backend_theme_v10

enter developer mode in your Odoo10 interface which is located in Settings on the right-hand side under “share the love.” Click on “Browse Apps” which is automatically filtered to include only “apps” so clear that from the search area and search for the name of the theme you installed. In this example, it’s Material/United Backend Theme.

Click Install on the theme and you’re finished! Now you can use git to check for updates to the theme or have them automatically pulled down with a chron job. You’ll need to hit the upgrade button to apply any pulled updates.