So, a further update, I’ve found and fixed a number of places in the theme which explicitly load http:// resources rather than https:// ones. I think there was one js one file in the Content Builder part of the framework, and several calling into Google fonts.
So the incorrect layout of the Page Builder tabs is now fixed and they do now appear as tabs.
I’m now seeing only one remaining error in the Browser console:
GET https://www.domain.com/wp-content/themes/circle-flip/style.php?is_rtl=0&ver=4.4.2 fails with a 403 error
That seems a bit odd – I’ve checked the file permissions, which are rwx r-x r-x which is the same for all the other php files in the theme folder, and the user and group are set similarly. Looking at the content of the file (albeit only briefly), it seems this could be important 😉