Comment 38 for bug 625193

The purple and orange are nice colors for a theme.... but when you put them on a gradient, they mix together. Purple+orange mixed gives you a very unappetizing brownish color. If you could come up with a Purple+orange+brown theme that worked, this would be okay. The gradient however adds colors to the overall theme that are just don't belong in the Purple+Orange.

That, I think, is the unintended consequence of the design of this background, which is what makes it a bug, and not just an aesthetic difference of opinion: the brown is (I think) not intentional.