Arc is a flat theme with transparent elements for GTK 3, GTK 2 and Gnome-Shell which supports GTK 3 and GTK 2 based desktop environments like Gnome, Unity, Budgie, Pantheon, XFCE, Mate, etc.
This is a companion discussion topic for the original entry at https://copr.fedorainfracloud.org/coprs/tesseract/arc-theme/