When comparing Iris Light vs OneStepBack, the Slant community recommends OneStepBack for most people. In the question“What are the best gtk3 themes?” OneStepBack is ranked 7th while Iris Light is ranked 25th. The most important reason people chose OneStepBack is:
The color combination has been carefully engineered exactly for the purpose of making people comfortable, while staring at the monitor for a prolonged period of time.
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Pros
Pro Multi GTK support
Iris supports newer versions of GTK3 (3.10) and also GTK2.
Pro Easy on the eyes
The color combination has been carefully engineered exactly for the purpose of making people comfortable, while staring at the monitor for a prolonged period of time.
Pro Allows you to focus on what's needed
The design is extremely basic - using just 1 color accompanied by 3 shades of grey, OneStepBack manages to be a very functional theme without adding any flashy.
Cons
Con Development not finished
Iris is still in beta so there may still be bugs and other glitches.
Con Looks old and dated
it's not flat
Con Funky arrow button colors in FireFox
The developer is currently aware of this issue, however the fix can't be made until FireFox has an update.