Problem I've found is that there aren't any simple GUIs for creating/tweaking GTK3 themes. You could configure the colour scheme slightly (5 colours) with Gnome 2 and the older GTK engine. The other thing, is that you still have a mix of GTK3, GTK2 and non GTK apps, well at least I think that's true. OpenOffice being an example. It's an annoying problem for any distro, but the end result is that you end up with loads of inconsistency.

In short it's more of a pig for a user to configure their desktop style than it was in Windows 98.

So yes you could swap your theme, but I have yet to find many professional themes, and even the better ones like Bluebird, have issues.