I figured I might add my $0.02 to this discussion. While I certainly can agree that the current web interface could look more sexy visually speaking, it is very easy to navigate and figure out what goes where in it. It requires very little mental effort to understand it, which is an unfortunately often ignored feature when many do web design. While visual appeal is nice, it should *always* be second priority to the functionality of the design.
So I would agree with colba, it might be a good idea to implement support for themes so that it is easier to evaluate and create new interfaces for proxmox, because I'd hate to lose the current one to a less functional interface without having a choice.