When comparing Orion vs DarK, the Slant community recommends DarK for most people. In the question“What are the best gtk3 themes?” DarK is ranked 8th while Orion is ranked 29th. 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.
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Pros
Pro Plug-ins including Pixlr image editor
Pro Built for front-end development
Orion was developed specifically for front-end development, making it a strong option for front-end developers thanks to features like syntax validation with ESLint and HTML content assist.
Pro Eclipse-like UI
Orion is developed by Eclipse and shares a lot in common with the Eclipse IDE UI.
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 No runtime environment
Does mostly do file editing, but without good project support, web terminal, build/runtime environment.
Con JavaScript only
Orion currently only supports JavaScript.
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