I meant it tongue-in-cheek, but what you say is true and the Steam interface has always been a custom one. I haven't used the Mac version - how did they rectify this on the Mac?
The Steam UI on the Mac looks exactly like it does on Windows. They skinned the whole thing. And this isn't a bad thing. Steam is intended to be a single cohesive gaming platform. It should look the same on every OS that it runs on.