Hello and welcome to the forum
Probably nothing.
IF you're talking about the default Desktop Background (the one with the four windows middle right with the radiating light affect).
At some point Microsoft fiddled with the color, hues and tint changing it slightly which makes it appear brighter and less attractive (if that's possible). Many people have commented negatively about that change.
Have you attempted to change it by selecting either one of the other "Picture" choices available by default or perhaps a personal image present on your computer just to see if you see the same issue?
IF your problem is only with the Desktop Background and you are not experiencing anything similar when using your computer generally in other programs or Apps, my suggestion is to experiment with other images until your find something a bit more pleasing to the eye.