When comparing Flat Remix GTK theme vs DarK, the Slant community recommends Flat Remix GTK theme for most people. In the question“What are the best gtk3 themes?” Flat Remix GTK theme is ranked 5th while DarK is ranked 8th. The most important reason people chose Flat Remix GTK theme is:
The additional color options are all very well thought out, too.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Very clean look
The additional color options are all very well thought out, too.
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 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