When comparing Elegance Colors vs DarK, the Slant community recommends DarK for most people. In the question“What are the best gtk3 themes?” DarK is ranked 8th while Elegance Colors is ranked 18th. 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 PPA for Ubuntu
There is a PPA available for Ubuntu users, which makes installing a breeze.
sudo add-apt-repository ppa:satyajit-happy/themes
sudo apt-get update && sudo apt-get install gnome-shell-theme-elegance-colors
Pro Changes color
Elegance Colors can change it's Gnome Shell theme colors in accordance to the users GTK them, wallpaper or a user defined color.
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Con Outdated
The theme project has been abandoned and there has been no activity for the past two years (the last commit was in May 2015).
Con More involved to set up than other themes
Users need to run a command to run a process in the background for the theme to work correctly, which may be more involved than what users are typically looking for in a theme.
Con Doesn't work
Doesn't install and doesn't work.

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
