[mrweasel]

I can understand people complaining. The dialog UI you linked to is a horrible grey text and grey, it's really hard to read. The rest seems to be a case of "flatten it, because it's modern", it really don't make thing more visible or clearer.

[smrtinsert]

I find it perfectly fine to read and it seems way less cluttered.

[steveax]

Colors in the GUI affect your perception of colors in the image you're working on. I suspect that was the motivation.