If you look on https://www.steamdeck.com/en/software you can see that the home screen looks different with a colored background and blurred “taskbar”.
And if you scroll down you can also find that the library has a very different feel.
This can also be seen in this videos https://www.youtube.com/watch?v=WfoLRmKwnSI B-Roll of the Limited Edition OLED.
Thats cool and I can confirm that its apart of the Steam Beta and not SteamOS 3.5.x. I think the QAM and “Steam” menus could do with a slight redo to fit better with the new theme but otherwise I think the Home Menu, Library and Settings look great compared to before! Most colors seem to be a bit darker than before so thats nice for us who like dark themes.