Not sure why you were downvoted...

Making buttons that melt into the background is demonstrably bad accessibility.

Also, the default window background colour now looks like a washed out water colour brown.

That's just a misunderstanding. It isn't a background colour, it's a glassy effect texture. So the "brown" is just from whatever the desktop background image was behind the window. If the second screenshot had the same desktop background and window accent tinting settings then the backgrounds would have appeared the same between screenshots.