I have created instructions for improving the appearance of GTK3 apps that use "client side decorations" in Plasma 5. Here is a link to the tutorial. This fixes the issues with CSD apps having no border, which makes it hard to resize the app, not to mention it looks ugly. I've also fixed the duplicated window title text and improved the header bar color to make CSD apps fit better into the Plasma 5 ecosystem.

Cool tip!

I also had to search around on how to make scrollbar wider. GTK themes can be a pain.

BTW, I recommend you Oomox to make your own GTK themes.