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