decoration method
Implementation
Decoration? decoration(Decoration? delegate) =>
delegate.selfOrDefault(BoxDecoration(
borderRadius: theme.borderRadius,
border: Border.all(color: theme.borderColor),
color: theme.backgroundColor,
boxShadow: [
BoxShadow(
color: theme.shadowColor,
offset: theme.shadowOffset, //(x,y)
blurRadius: theme.shadowBlurRadius,
),
],
));