In Flutter, each property and value is specified separately.
Use the boxShadow property of BoxDecoration to create a list of
BoxShadow widgets. You can define one or multiple
BoxShadow widgets, which can be stacked
to customize the shadow depth, color, and so on.

box-shadow: 0 2px 4px rgba(0, 0, 0, 0.8),
              0 6px 20px rgba(0, 0, 0, 0.5);
}