...I'm sorry I'm not a theme designer; but if someone's got free time; do post a mockup of how Jonny Ive would design a theme for xenForo. Remember - be as flat as you can.
[...and then we'll make a video with guy on a white background that says ... "WOW"]
I think Xenforo the way it is would be pretty close. Clean, simple interface without useless clutter. Seriously, I bet if you asked Ive what he thought of Xenforo compared to other forums and he'd say it was closest to how he'd do it.
I might put together a simple Helvetica Neue-based flat theme at some point - I do very much like the iOS 7 aesthetics. I can certainly see how Jony Ive would make a XenForo theme.