How to purge cached Diazo theme data for non logged in visitors in Plone5 after TTW edits

Question

I freshly uploaded a Diazo Theme through the Plone5.0.2 theming control panel to replace an existing version of the same theme.

The theme imports Barceloneta and settles on top of it.

After recompiling the main less file calling Barceloneta and the additions, everything works fine in the current session including the edit UI for a logged in user. Even after local TTW edits and recompiling and Clear Cache everything is reflected at once.

But the content for unauthorized users stays cached and the former theme survives all kind of attempts to purge it from browser and Plone caches.

I pressed all the buttons in

• Diazo Theming, Theme Panel, Modify Theming Editor
• Resource Registry panel (Development mode on/off and saved!)
• Caching panel (RAM cache purge, disable caching in general)
• Browsers.

I access the site bypassing any nginx, ha-proxy or varnish setups via a tunnel directly.

Is there an unobvious method I missed?

I can list all detailed steps on demand.