The Plone CMS/DMS came with a theming editor, where you could create a new theme and edit themes. You could copy an already on the site available theme or create a theme from scratch. It’s your choice. I used the theme part in the administration interface to copy the Plone default theme ‘Barceloneta’ and save it under a new name.
I could edit this new theme with the Plone theme editor (button ‘Modify Theme’.
The theme is based on the Diazo theme engine (https://docs.plone.org/external/diazo/docs/index.html#diazo-theme-engine-guide)