SteamOS 3.5.5 has been released to the Stable channel for Deck with the following changes: Display The default color rendering for Steam Deck LCD has been adjusted to emulate the sRGB color gamut, resulting in a slightly warmer and more vibrant color appearance. Added Settings -> Adjust Display Colors, to tune the display's Color Vibrancy and Color Temperature. The settings can be tuned with a preview of a test image (above), or with your running game.
What about Nested Desktop?
It shows up as an application in the menu for KDE, just right click it and select “Add to Steam” and it will work as desired on the Big Picture mode.