If I understand it correctly following this tutorial you create new plone installation with the custom theme in it.
I was windering if i cannot just create the custom theme based in abrcelone in an existing Plone
If you want to learn how to make a theme (and do it from 'scratch'), you follow the tutorial
If you 'just want to make a theme', you do the 'mr.bob' thing and put the add on in src (folder) and add it to buildout (in eggs and develop).
If that also is complicated, you duplicate the barceloneta theme in the @@theming-controlpanel and edit it TTW (and/or export it to use it on other sites and as a backup)
If you just wants to make small edits to existing themes you can duplicate them in the theming control panel and give them the same name as the old.