When comparing Iris Light vs Arc, the Slant community recommends Arc for most people. In the question“What are the best gtk3 themes?” Arc is ranked 1st while Iris Light is ranked 16th.
Ranked in these QuestionsQuestion Ranking
Pro Multi GTK support
Iris supports newer versions of GTK3 (3.10) and also GTK2.
Pro Customizable to dark or light themes
Pro Has both GTK2 and GTK 3 support
Users will be able to match their gtk2 and gtk3 themes easily due to Ark supporting both.
Pro Supports gnome-shell
Pro Available firefox and chromium themes
There are matching themes for Firefox and Chromium for those that want to have a fully matched looking environment.
Pro Really clean and nice-looking
Con Development not finished
Iris is still in beta so there may still be bugs and other glitches.
Con Bleak font colors that cause eye strain
The font color in Arc and Arc-Darker is not black, but lightgray
after prolonged usage, it becomes apparent that this font color, combined with white backgrounds causes too much eye strain
Con Has some contrast issues
The theme is a low contrast theme so it really depends on the screen brightness if you see borders or the text, some elements don't even have borders - this makes the theme barely usable for portable devices.