System Settings/Widget Style
This module is used to configure how the individual widgets are drawn by Plasma.
Note
A Widget is a commonly-used programmer's term for referring to User Interface elements such as buttons, menus, and scroll bars. You can think of them as the fundamental pieces that are assembled to make your application.
These settings only apply for Qt applications. Look at System Settings/GNOME Application Style (GTK) to configure the theme of GTK application. If you are interested in better integration for GTK app, look at the list of KDE themes with GTK ports.