Apologies if someone has already made this. I am surely not the first person to have thought of this.
TNG S2E16 “Q Who”
https://store.steampowered.com/sale/steammachine
EDIT:
TOS S3E5 “Is There in Truth No Beauty?”
https://store.steampowered.com/sale/steamframe
So how complete an operating system is SteamOS? Does it have LCARS?
It is likely more trouble than it’s worth, and I think you would have to apply the change every time the device updates, but you could probably install it as a desktop environment to use instead of KDE Plasma.
https://lcarsde.github.io/index.html
For a much simpler/practical way to pretend you are using LCARS, there are LCARS-themed boot videos for the Steam Deck. I presume a separate version can be made that doesn’t use the Steam Deck logo like this one.
https://steamdeckrepo.com/post/YJO1P/star_trek_lcars_boot_sequence