The github page for Arc Icon Theme says you can change what icon theme it inherits by editing the index.theme file. I found this file in
/usr/share/icons/Arc-Icons and also
/home/<user>/arc-icon-theme/Arc, and have edited both to:
[Icon Theme] Name=Arc Inherits=Numix Circle,Numix,Adwaita,gnome Comment=Arc Icon theme ...
However, when I select the Arc Icons with Unity Tweak Tool, it shows the Arc Icons correctly but defaults the rest to the system's default icons. Numix Circle works fine on it's own, too.
I'm new to Linux so I'm probably missing something pretty simple.