Ah right. So it’s adw-gtk3 theme. When I toggle support for GTK apps in COSMIC settings. It’s just writes to gsettings adw-gtk3 as a theme and modifying gtk.css then.
I notice also. Snapd prompts for installing missing themes when this toggle is enabled 
I had this working somehow, before. Please look this post
Not sure to be honest, how I got this working tho 
I tried to refresh few snaps, it might be snapd-desktop-integration + snapd itself. But it’s not working this time. Or something changed to adw-gtk3 or something else.