When comparing DarK vs Paper, the Slant community recommends DarK for most people. In the question“What are the best gtk3 themes?” DarK is ranked 8th while Paper is ranked 19th. 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.
Ranked in these QuestionsQuestion Ranking
Pros
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.
Pro Flat and colorful design
The look of this theme is inspired by Material Design.
Pro Does total makeover of your computer
It's a complete theme, with custom icons, GTK, cursors, Gnome Shell.
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
Con There is jumpy behavior when switching from window to window
You can notice jumpy behavior when switching from a forefront window to window behind it. Both windows jump a little which can slightly disrupt normal working flow.