Hi there,
There is no built-in functionality for changing the default breakpoints in the Storefront theme. The best way at the moment would be to dequeue the theme CSS file and then copy the contents into a custom stylesheet where you can change the breakpoints, and then enqueue that style.css file instead.
There’s an enhancement request for adding an easier way to change breakpoints in the Storefront repository here:
https://github.com/woocommerce/storefront/issues/991
That said, this is the plugin for the core WooCommerce plugin. Storefront has its own forum available at the link below, so for any other Storefront-related questions, please post your question there:
https://wordpress.org/support/theme/storefront/
I hope that helps! Have a wonderful day!
