You can create "soft" shadows for square or round objects that sit on a solid color background (see above). Make a copy of the object, then change its fill to a "gradient, from the center," shaded from black to the slide's background color, with the shading set with black going from the center out to the background color at the edges. Make this object about 150% bigger than the original object, and put it behind the object. This will give you the effect of "soft" shadows.