Change terminal tab color in Ubuntu 18.04

I am exactly looking for the same as these guys:

How to make selected tab in terminal more prominent
Customizing terminal tab color in Ubuntu 17.10

They use this file:

~/.config/gtk-3.0/gtk.css

But I can’t find it in my Kubuntu 18.04

Is there any new solution for 18.04 ?

Thanks

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Author: user204415