Accessing Checkout Theme Files
The files that control checkout’s text and styling live in the theme attached to your channel, not in your storefront code. Checkout runs on BigCommerce even when your storefront does not.
The control panel hides the Themes section on headless channels because most theme settings do not apply to a storefront you build and host yourself. You can still open the files directly by URL.
Open My Themes for a headless channel
To find your channel ID, open the channel in the control panel and read the numeric value that follows /channel/ in the address bar. From My Themes, select Edit Theme Files to browse the theme.
If Edit Theme Files is unavailable, the channel is using a read-only marketplace theme. Make a copy of the theme, then open Edit Theme Styles > Edit Theme Files on the copy. A copied theme stops receiving marketplace theme updates.