When comparing adapta-gtk-theme vs OneStepBack, the Slant community recommends adapta-gtk-theme for most people. In the question“What are the best gtk3 themes?” adapta-gtk-theme is ranked 1st while OneStepBack is ranked 7th.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Real dark theme
Pro Follows the material design guidelines
Pro Easy installation on Ubuntu
sudo add-apt-repository ppa:tista/adapta
sudo apt-get update
sudo apt-get install adapta-gtk-theme
Pro Most complete theme
It styles every nook and cranny of the OS UI, even the ugly Java apps look beautiful using this theme.
Pro Ergonomic
Not only looks good, but really accelerates work on desktop.
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 No Debian support (sudo apt-get install adapta-gtk-theme)
Con Pertamax
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.