When comparing Papirus vs DarK, the Slant community recommends Papirus for most people. In the question“What are the best gtk3 themes?” Papirus is ranked 3rd while DarK is ranked 8th. The most important reason people chose Papirus is:
All icons look pleasing, especially with the Arc theme.
Ranked in these QuestionsQuestion Ranking
Pros
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
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 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.
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