First you need to mount your website's filesystem.
If you have a MAC go to Finder and then click on Go->Connect to Server. If you have a PC, right click on the Network icon and click on 'Map network drive'.
Use the url: https://<your_drupal_website_url>/_site/files
Then you need to have Git installed. If you have a MAC you need to install the Xcode tools and run them once (this will install Git for you).
If you have a PC then you need to download and install Git from here.
Then go to your website's themes directory and clone the theme by typing in terminal:
git clone https://gitlab.cern.ch/webteam/cern-overwrite.git cern_overwrite
In order to clone the theme you must be a member of either the it-dep-entice or drupal-users e-group.
You can visit the e-groups' page on https://e-groups.cern.ch and send a subscription request.
After you have successfully cloned the theme, go to your Drupal website and enable it under 'Appearance' menu.