Amazon's been testing a new design for its homepage, and it appears that this rather dark-and-stormy front page has today been appearing for far more users across the US.
Amazon has apparently been working on a design refresh since July. Now, it seems Amazon is selectively using this new front page on a variety of operating systems and browsers. The changes aren't huge: the dominant one is a flat dark bar across the top of the page, with a new dropdown menu for Fire and Kindle products. Product pages remain unchanged.
This appears to be isolated to the US Amazon site. Any changes wouldn't necessarily be instantly applicable to the UK site, as the design of the two differs somewhat.