Usually that color has the RGB values 192,192,192 and I haven't found
a way to modify it. Not connected somehow to the Excel Color Palette
and not connected to the Windws colors (Appearance settings). A long
time ago I found a vba code snippet how to change this background
color for the MS Access application via some windows API calls, but I
never figured out whether the same can be achieved somehow for the
excel window background. Not a trivial one.