Beta testers like diging around in files, and it looks like some of them found hits of a new Steam UI
Steam has changed a lot since its launch back in 2003. A lot of features and functionality was added over the years, usually on top of each other or squeezed somewhere in between. A UI redesign is actually quite over due, and while Steam Big Picture might be fine for controller-centric use, it is less than optimal when a mouse and keyboard come into play.
Files and references to 'SteamU' are supposedly packaged inside the latest Steam beta and what is on display looks like an early design for a new Steam interface. In any case, the images are quite rough and probably just placeholders. If there is a new design in the works, it might actually still be years away from a proper release.