If I'm understanding the issue correct, you uploaded a custom wallpaper, and how you have grey bars that appear either on the sides or top/bottom?
There is no way to change the grey color, it appears as a filler if the wallpaper doesn't fully fit on the screen. It sounds as if the wallpaper is a different size or aspect ratio of the monitor the SX10 is connected to, if this is the case, edit the wallpaper to either fit the resolution of the screen being used, or add white to the image where you see the grey appear.