I think it's become clear to me, having spent about an hour on that site, that your choice of colors is really quite restricted if you're looking to style the entire OS.
If you're looking for something that would be easy to read, functional, universally appealing and that you wouldn't get tired of quickly, you're left with white (or shades of gray) for backgrounds and blue for accent colors. That's because black text on white backgrounds is generally easier to read & better for comprehension. As for accent colors, green is probably out as red-green is the most common type of color blindness. What are we left with then? Yellows? Purples?
So I guess it's no surprise that some of the better-looking ones tend to borrow heavily from OS X.