[i3wm] System scrollbars etc. in Firefox

Greetings.
As I've changed from Unity to i3wm^[1] I noticed that things like scrollbars and new tab sign have changed and so
I'm willing to change them to something that steals less attention but would like to do so from system/firefox configs and not from Firefox userstyles.
This is mainly about FF as scrollbars in Nautilus and Terminal are same as they were.

That is the only similar post that I've found - https://www.reddit.com/r/unixpo/co...my_scrollbars/ - but I don't know how to follow as those GTK folders are empty in my 16.04 and my skill would allow me to edit them only by changing values and I'm not sure what would it even affect, probably not FF?

^{[1] - (200 MiB vs 40 MiB vram at idle and my gc has 256MiB)}