"plastique" widget style disappeared after upgrade to kde5

[odin_ago]

After a recent system upgrade, if I understand correctly, KDE version became 5. When I restarted my computer, all of KDE desktop configuration has gone (I had to configure again many things, for example, icons in the system tray, keyboard layouts, power managment, etc). One of the things I cannot get back is various aspects of application appearance (for example: fonts, the way how the close button of a window looks).

Now I want to get back the appearance of scrollbars. In the new version, this is controlled by System settings -> Applicatioon style -> Widget style (configuration window) -> Widget style (drop-down list). In the previous version, this "widget style" option was set to "plastique", which looked as I like. Now I don't have it in the list anymore.

How do I get i back?