Hi, see the themes page linked on newsletter official page (www.satollo.net/plugins/newsletter): you should migrate your old themes to the new system. It’s easy it should take few minutes following the examples.
With the new theme system, you can create a per-theme configuration panel: this is way the update did not preserved the cmpatibility.
Your themes are inside newsletter-custom folder, you need to move them on wp-content/extensions/newsletter/emails/themes.
Anyway see the guide. I’m here to better support you on this transition.