Diazo theme is active in ZMI in Plone 5.2

[mauritsvanrees]

plone.app.theming has a patch to make sure that the ZMI is not themed. But on Plone 5.2 this does not work. The code patches some DTML files to insert a X-Theme-Disabled header. The patch is loaded and these files are patched. But Diazo is still active. I have seen this with a customer site on 5.2 where compiling Diazo takes several seconds per requests, and the ZMI has this problem too. Appending `?diazo.off=1' to the url fixes it.

Likely this is caused by Zope 4 having a new ZMI.