Steam says good bye to its old, but nice, look that was introduced about 5 years ago and says hello to a more memory using and modern look. The new look itself is not an official release - yet - rather a voluntary beta.
The beta can be accessed by clicking entering the Steam Settings and selecting Account. Click the "CHANGE" button and select the UI update, after that you will be asked to restart steam. After that you can see the Steam window changing while it updates. It will ask you to re-login so you should know your password for that matter.