Note that this doc refers to Shoptimizer for BigCommerce. If you are looking for the WooCommerce version go here.
If you’re making modifications to the core Shoptimizer for BigCommerce theme then we recommend using a child theme.
This child theme will work with any version of Shoptimizer for BigCommerce. You don’t need to update it if the parent theme is updated!
Small CSS tweaks
If however, you’re making just small CSS adjustments, these can go into Appearance > Customize > Additional CSS.