Barceloneta has a grunt-based build system, which is not really today's favorite tool for building frontend stuff, but still works fine. Inside the barceloneta folder, a first good step is explained in the HOWTO_DEVELOP.rst file, which basically tells you to
npm install and run grunt, but you still need a running Plone site to develop a theme.
There is an optional browser sync task in the Gruntfile which lets you have hot replacement of your styles while working on them.
One thing I did not understand from your question is: do you have any past experience with Plone, or is this the first time you are trying to install Plone?
The actual answer you are looking for may vary based on this
On the other hand, I'm looking right now into the barceloneta repo, it looks like there was an attempt at making it possible to develop it without a Plone site, but it's not working. I think that the index.html file in the project root is probably there for dev purposes. It has bugs (or it's just outdated) and it could use an update in the js dependencies.
For example, the link to the less files is broken, they are now inside
plonetheme/barceloneta/theme/less, and some less variables are missing or have simple typos (missing colons).