What are some alternatives?
When comparing Iris and Materia, you can also consider the following products
Moka - Moka is a Gnome Shell theme that uses simple geometry & basic colouring.
Eyeleo - EyeLeo is a handy PC application that reminds you to take a break for your eyes.
adapta-gtk-theme - An adaptive Gtk+ theme based on Material Design Guidelines - adapta-project/adapta-gtk-theme
Dark - Holistic language, editor, and infra for building backends
Arc - Arc brings your favorite games, communities, media and
Zukitwo - Zukitwo is a Gnome Shell and GTK theme that currently works on Gnome Shell 3.14 and the latest version of GTK.