I'm helping someone develop a website in BigCartel right now. I'm able to make changes to my CSS code when I go to the Design option, but I can't make any changes to my HTML code.
I tried right-clicking to bring up the dropdown menu, and clicking Inspect to try and change the HTML that way. But that didn't work.
To edit the HTML, you have to go into Design, then to the Code tab on the top right of the editor, and you'll find it broken up into different sections. The Layout option is the overall code on the site, or rather the framework, while the other options below it will contain the HTML for different pages. However, if you're on the free plan, you won't have permission to edit the HTML unless you upgrade to the Platinum plan or higher.