When comparing clearlooks-phenix vs Papirus, the Slant community recommends Papirus for most people. In the question“What are the best gtk3 themes?” Papirus is ranked 3rd while clearlooks-phenix is ranked 11th. The most important reason people chose Papirus is:
All icons look pleasing, especially with the Arc theme.
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Pros
Pro Good contrast
This theme has a good text and border contrast.
Pro Available everywhere
Clearlooks was basically ported to almost every OS or device so you can even make your virtual guest OS look like your host OS.
Pro Sharp, flat, and fresh
All icons look pleasing, especially with the Arc theme.
Pro Familiar
Used on Lubuntu and BunsenLabs, for example.
Pro Active development
Cons
Con Under GNU GPL3
This limits the use elsewhere.
Con Contrast issues / No border around icons
It makes the the fast and makes it simple to add more icons(since you can simply scale them) however the contrast is lost in some situations.
Con No Ideas
It just copies the icon-syle from a different system.
Con Just another material style copy
Con Lacks icon for backintime
Backintime tray icon doesn't display on the panel because Papirus lacks the icon.