I am using the dugway CLI tool to develop my theme. When I do dugway build, it creates a zip file with my bundled theme.js javascript code and an image folder with all my images, I'm referencing in my html files.
But on my Bigcartel account I can only copy in the HTML files and my CSS styles manually via the advanced functionality.
How do I upload my JS file and my images?
Unfortunately, Big Cartel can't host any files such as javascript. If you can find a way to host those files elsewhere, you should be able to link to those through the custom layout page and be able to use your custom theme.