WordPress registration page has no styling

Thanks to help here and elsewhere, I’m making good progress using Customizr with the Marketpress eCommerce plugin. This post was a real breakthrough:
https://www.ads-software.com/support/topic/marketpress-wpmu-and-customizr

However, my WordPress registration page has no styling. When wp-signup.php is called there is no sign of the Customizr theme, only page content. Twenty-thirteen theme loads the registration page no problem.

I’ve followed dozens of leads over the last few weeks but have not found a simple solution. For now, I’m using the Theme My Login plugin and a 301 redirect in .htaccess to a Register page on my Customizr site. There must be a better way.