Arc-gtk-theme source on Arch repo has changed from horst3180 to nicohood.
Nico provides us with Openbox support out-of-the-box. Horst did not.
We can delete this package for the installation to continue. We do not need it anymore now.
Thank you Nico for supporting Openbox in the arc-gtk-theme package.
sudo pacman -R openbox-arc-git