The background start color. This is the color that is drawn at
the top or left of the view, depending on the value of
backgroundGradientOrientation. If this and the backround stop color
are specified, a gradient background is drawn in the direction specified
by backgroundGradientOrientation. If only one of the background start
and stop color are specified, a gradient of the same color that fades to
transparent at the unspecified end is drawn. If a box shadow is drawn,
the background defaults to white. Specify a start and stop opacity to
suppress this behavior.