The "themes" folder holds other projects that your project will inherit assets, content,and layouts from. You can add, remove, enabledisable, and update themes through the CLI.

Every Plenti website can be used as a theme - in fact there is nothing special you would doto your site to make it a "theme." Simply nest any Plenti site (if you have permission) in the "themes" folder of your project and specify its folder name as the theme key in plenti.json.

Omitting assets, content, or layouts from a theme

By default you inherit everything from the parent theme, but you can choose not to inherit particular aspects if you'd like. For example, you may want to use components from the "layouts" folder, but don't want to use any of the content or images. You could do that in plenti.json like the following example (uses this website as the theme):
  "theme": "",
  "theme_config": {
    "": {
      "url": "",
      "commit": "0eff8c7",
      "exclude": ["content", "assets"]