System Settings >> Workspace theme >> Look and feel/Desktop theme show a blank page. That means the user is unable to change all of the related settings because they are unavailable. Workaround. Install the qml-module-org-kde-kcm package, in Neon is sudo apt install qml-module-org-kde-kcm, this solves the problem and the options are both available and work as expected (you can change desktop themes etc). Fix. Mark the qml-module-org-kde-kcm package as a dependency for the systemsettings package and make it installed by default. This is likely caused by a recent change in KDE Frameworks lately since I haven't encountered this bug before.
*** Bug 389112 has been marked as a duplicate of this bug. ***
*** This bug has been marked as a duplicate of bug 387689 ***