Yep, this happens in Squarespace 7.1. The themes you switch between in Site Styles aren’t real templates, they’re just presets. So when you switch from preset A to B and then back to A, Squarespace resets all your custom fonts, colors, and style choices.

You cannot really. Squarespace doesn’t let you save your own preset. And you can only export the Custom CSS you manually added, not the full Site Styles settings.

Pick one preset and stick with it, then use Custom CSS for anything you don’t want to lose. Also you can try Squarespace speed optimization tips for better performance.