Q:

how to change the opacity of the snackbar in flutter?

I wish to change the opacity of the SnackBar. It has only the background property. Can it be customized or I have to create the custom widget for snack bar? 

A:

Try using the color property of snack bar like this,
  backgroundColor: Colors.black.withOpacity(0.5)

This should work as you expected.

A:

You can adjust the opactiy of your backgroundColor with 

color.withAlpha(..), 
color.withOpacity(..), 
using a hexadecimal integer 0x33ffffff (the first pair of digits after the x represents the alpha value),
creating a Color using Color.fromARGB(...)
or by using Color.fromRGBO(...). 

You can find information about this on this documentation page about the Color class.
Now, you face the following problem: Your content is not yet translucent. 
This is easily adjustable using the Opcacity Widget.
In your Snackbar just surround your actual content with an Opacity Widget:
SnackBar(backgroundColor: Color(0x66bbbbbb),
  content: Opacity(opacity: .7,
                    child: Container(), // your content
  ),
)

