I just got back from my holiday and both of my Roku devices have an updated desktop. Does that mean there is been a recent update?
If its just the background that changed, then it was probably a theme change rather than an OS change. Some people turn off the seasonal background changes because it bugs them when that happens.
By updated desktop, do you mean the Wallpaper changed or you now see different menu items or Search bars/Continue Watching features on your Home Screen?
If simply a different wallpaper, try going to Settings/Theme/Seasonal Wallpapers and selecting [Off] to disable them if you do not want the wallpapers to change.
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Users should still be on Roku OS 12, but some features like Continue Watching seem to be being added to user devices short of a regularly released update (ie. 12.5). Others users are experiencing some trial features being tested on their devices like an added Search Bar or Bluetooth tile.
You can see what OS build you are on through Settings/System/About and determine when the device was last updated through Settings/System/Software Update. (look under Last Updated. This will tell you Software Version and build number as well).