How to amend the Barceloneta theme with a bit of CSS

What is the best way to slightly modify the Barceloneta theme using additional CSS? The section on portal_css in the manual says that Plone’s resource registries, including the portal_css tool, can be used to manage CSS stylesheets. Is this possible for the Barceloneta theme? If yes, how exactly is this done?

Good catch, you probably refer to the "Advanced: Using portal_css to manage your CSS" section. This has been copied from plone 4, but portal_css and portal_javascripts are deprecated in Plone 5.

For detailed instruction on theming with in Plone 5 and using the new resource registries the material on the theming training is more up to date:

Wow, our documentation refers to the deprecated portal_css? Thanks for pointing this out. Do not go this path!

If you have your own theme easiest is to use the theme manifest.cfg to add your own CSS. This looks like so:

development-css =  /++theme++MYTHEME/css/theme.css
production-css = /++theme++MYTHEME/css/theme.min.css

Thank you for this link.

Generally, it would be good to make it easier to find such documentation. For example, I had been completely unaware of the site.

The point is that I would like to amend the Barceloneta theme. So should I copy the Barceloneta theme and then make these changes to manifest.cfg and add the corresponding CSS files?

So I should not use portal_css, but the page that fredvd referred to says in the first section that the Resource Registries control panel should be used, which I thought is referring to portal_css and friends.

There is an old registry portal_css (deprecated, file bugs if it mentioned in the docs anywhere except that its deprecated) and a new one in the control panel. But for theme only development its imho easier to use the manifest. I do not know the rational behind the way its described in the docs, maybe @fredvd followup here.