As you are aware, the KDE Plasma 5 desktop environment uses Qt5 technologies, and while there are some tools to make GTK apps look great on Plasma 5, you can’t use GTK themes per se. Right now, Adapta appears to be one of the most popular GTK themes, and you can now finally use it on your KDE Plasma 5 desktop. The Papirus development team took care of the porting of the popular Adapta GTK theme for KDE Plasma 5 desktops, and they also managed to add some extra things to make your experience even better than if you were running a GTK-based desktop environment, such as GNOME or Xfce.