When comparing Iris vs DarK, the Slant community recommends DarK for most people. In the question“What are the best gtk3 themes?” DarK is ranked 8th while Iris is ranked 10th. The most important reason people chose DarK is:
The theme fully supports the GTK3-nocsd hack so GNOME3-apps will integrate well into XFCE or MATE.
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Pros
Pro Purchasable filters to heighten visibility in certain conditions
Iris has filters that can be purchased through in app purchases that allow for increased visibility in certain lighting situations.
Pro Easily turned on and off
Iris has a quick toggle in the notification drawer with allows for the user to easily turn on and off the function of the app.
Pro Puts an overlay of your camera feed on top of all other apps
By overlaying what the camera sees over any app on the device, users can see what is in front of them while still looking at their screen, retaining all the functionality of every app or game they use.
Pro Fully supports GTK3-nocsd
The theme fully supports the GTK3-nocsd hack so GNOME3-apps will integrate well into XFCE or MATE.
Pro The color can be easily changed
A script is included to change the highlight-color to almost everything.
Pro GTK2 and GTK3
Supports both GTK2 and GTK3.
Pro Has full HiDPI support
The theme and all included sub-themes have full HiDPI support.
Cons

Con Seems to not be developed anymore
Link is not working and it is not possible to find an OnyX theme for GTK3 anywhere.
Con No-GNOME etc. support
As it is made to support traditional desktop environments so there is no support for GNOME3 and its forks
Con Dark theme
It's a dark theme, so you might encounter the same issues as with all dark themes
