The most likely cause will be a hard-linked image in either your content or a template. To see which image is loading over https: in Firefox, press ctrl-i to bring up the Page Info window. Click on the Media icon and scroll down until you see a URL which starts with http. Then find out which content or template is responsible for loading that image.
yes, the templates and files with problems are:
++plone++production/++unique++2019-11-04T11:56:00.608217/default.css
++theme++barceloneta/less/barceloneta-compiled.css
However, I don't know how to overcome the problem.
The two files you list are static resources, see https://pypi.org/project/plone.resource/#files-in-a-central-resource-directory These can be either stored in the ZODB or on your filesystem. Are other static resources served correctly over HTTPS? If so, then it is very likely that you will find a problem in your theme.