You will need Go, so you can rebuild the binary. Then look at “./build.sh guidev” in building syncthing. This will give you a binary that uses the
gui directory in the current working directory instead of compiled in assets, so you can edit the html/css/js and just reload in the browser.
So pretty much as you assumed, except the binary has to be built for it.
When happy with a change, “./build.sh assets” to recreate the compiled-in stuff, then commit and pull request etc.