When applying a new theme It shows a DBus backend error after 25-30 seconds. The reason for this is that the Kauth call to the helper uses the default timeout. However helper itself has a timeout of 60seconds for the update-initramfs -u command. We need to set a timeout for the Kauth action to at least 60seconds aswell then. Though in general having a UI frozen for 60 seconds is a bit odd aswell. So it needs a progressbar or at least a Hint that shows it is working (Like a big overlay fading in ontop of the kcm showing a label "Working ... ")
Proposed patch now available here: https://phabricator.kde.org/D5943
That patch landed years ago; marking as fixed.