SystemColor constructor
Creates an instance of a system color.
name is the name of the color. System colors provided by SystemColorPalette, such as
SystemColorPalette.accentColor and SystemColorPalette.buttonText, use standard names
defined by the W3C CSS specification.
value is the color value, if this color name is supported, and null if
it's unsupported.
Implementation
const SystemColor({required this.name, this.value});