I’ve been testing several themes. I like the Matcha one and I’ve heard is the one manjaro dev will support. Which is great. However I would like to recommend another theme that comes in so many flavors: Vimix https://github.com/vinceliuice/vimix-gtk-themes - currently you can find it in AUR and from my tests it works and looks great. And the fact that it has so many flavors it makes it, in my view, the perfect candidate for the default Manjaro Gnome theme.
Vimix is a flat Material Design theme for GTK 3, GTK 2 and Gnome-Shell which supports GTK 3 and GTK 2 based desktop environments like Gnome, Unity, Budgie, Pantheon, XFCE, Mate, etc.