I discovered something today, that probably everyone knows, but was news to me and might be very useful to KDE Plasma 5 users who occasionally run the odd piece of GTK-based software.
As was the case with the KDE 4 default oxygen theme, there is now a GTK version of Breeze light and dark (the themes shipped by default with Plasma 5), called gnome-breeze. I think they make modern GTK software look very good on KDE, even better than the oxygen GTK theme once did!
Full details (for arch-based linux distros) and a screenshot below. Hope someone finds it useful
Arch wiki info: https://wiki.archlinux.org/index.php/Uniform_Look_for_Qt_and_GTK_Applications#Styles_for_both_Qt_and_GTK.2B