So today, I got request from client to modify the checkout page. It shouldn't be a problem of course. But, this "Optimized One-Page Checkout" is a stand alone page and I can't access the files from WebDAV. If your based theme is not Classic Next there will be customizer for it. But my base theme is Classic Next.
BigCommerce's doc, for me (again for me), is not so user-friendly. And because BigCommerce is not free, so the community is not so crowd. It is hard to find a thing. So after sometime searching everywhere, which in the end lead me to the official doc. So basically I can modify the styling of the page, with css file named `optimized-checkout-webdav.css`. And you need to upload it under `/content/`.