The plone.app.theming manual says that Plone ships with an example theme. However, this example theme does not seem to be part of my Plone 5.0.6 installation, since the only theme I can see in the theme overview is the Barceloneta theme. So where can I find this example theme?
There used to be a Twitter bootstrap example theme, but you can find other Plone 5 themes on the https://plone.org/download page
(filed an issue https://github.com/plone/plone.app.theming/issues/120)