If you’re making heavy modifications to the core theme then yes, we’d recommend doing so. You can download a child theme for Shoptimizer here.
This child theme will work with any version of Shoptimizer. You don’t need to update it if the parent theme is updated!
A child theme is treated by WordPress as a completely separate entity. So your customizer settings are not carried over when you activate one.
This is easy to solve with the Customizer Export/Import plugin. Upon installing the plugin you’ll see a new option in the customizer to import and export settings.
If you only need to make minor CSS tweaks
If however, you’re making just small CSS adjustments, these can go into
Appearance > Customize > Additional CSS.