Where can I download an old version of the Customizr theme ?
On Github
On WordPress.org
You can find and download all the previous versions of the Customizr theme in the WordPress.org platform.
For example, if you need to download v3.4.20, use this link : https://downloads.wordpress.org/theme/customizr.3.4.20.zip.
You can change the version number (3.4.20 in the example) by the one you need.
If you don't know which version you want, the version history is available here : https://wordpress.org/themes/customizr/