Sun Valley is the name for the upcoming redesign of the Windows 10 UI. We do not know much at this time, as Microsoft is trying to keep secret the new features of the next major release.
From time to time we have seen new icons, we've heard of rounded corners of Windows and a slightly more refined look in embedded applications.
Earlier this week, Microsoft announced that "Windows of the next" is coming as Satya Nadella mentioned in Build 2021. The presentation of the new operating system will take place on June 24 and the company will probably not be able to stop the internal leaks although he seems to be trying. So we already know that the next version of Windows will be called Windows 11.
An image leaked from build 22395 shows us the new design of the Settings application with new colorful icons and a renewed look in the categories.
According to Russian website leaked images, (this is a reliable source of information), the screenshot is genuine and real. They managed to confirm this with people inside Microsoft.
As for the "Settings" application, the main categories seem to have moved to the left and will have colorful icons.
It is worth noting that the screenshot is from build 22395 and rs_onecore_wsd branch, which does not currently exist, as Microsoft decided to hide the changes brought by the Sun Valley update.