At this time, the mobile breakpoint is located inside the /css/mobile.css file. It’s set to 768px.
I’m currently testing a way to include this breakpoint when enqueuing the file, meaning users could potentially dequeue the file set in the theme, and enqueue it again (the same file) using their own custom breakpoint.
However, for now, you’ll need to edit the core mobile.css file, and remember to make the change again when you update the theme (if the new feature above isn’t already included).
Hope this helps 🙂