It changed my KDE experience! Many thanks to the author!

I think the problem is that that window is partially using GTK and thus getting default (light) GTK colours.

Go to System Settings -> Application Style -> GNOME Application Style (GTK) and switch your GTK2 and 3 themes to a dark one. Optimally a GTK version of breeze dark, which definitely does exist (look for breeze-gtk in your repos, or else have a search around online)