How to add a margin or padding to a Snackbar?

Flutter team have updated the snackbar to match the material design in this PR. You can simply get the new behavior by setting

behavior: SnackBarBehavior.floating

Here is a sample code

final snackBar = SnackBar(
  elevation: 6.0,
  backgroundColor: Configs.current.COLORS_PRIMARY,
  behavior: SnackBarBehavior.floating,
  content: Text(
    "Snack bar test",
    style: TextStyle(color: Colors.white),
  ),
);

and the result will look like this

Not sure about margins. Round corner SnackBar can be created like:

Scaffold
    .of(context)
    .showSnackBar(
        SnackBar(
            content: Text(message),
            shape: RoundedRectangleBorder(
                borderRadius: BorderRadius.all(Radius.circular(20)))));

Use required border radius in above.

Update: You can use floating SnackBar to add default margins. Pass below to SnackBar constructor:

Scaffold
.of(context)
.showSnackBar(
    SnackBar(
        content: Text(message),
        behavior: SnackBarBehavior.floating,
        shape: RoundedRectangleBorder(
            borderRadius: BorderRadius.all(Radius.circular(20)))));