Thanks for your patience and help. I think the issue had something to do with Custom CSS rules, which is difficult to test all variations, and also not a very commonly used feature of the theme except by our more advanced users.
The whole backward compatibility issue is very difficult, and frankly, the Custom CSS is one of the hardest. I personally never use it, but instead just use the single global Custom CSS (either the theme’s version, or the built-in WP version which is really good when used with the Customizer and immediate screen updates.)
The theme will get a full update today as a stray letter got added that resulted in an undefined function which causes a crash while using the Customizer in What mode.