Where can I download an old version of the Customizr theme ?
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/