I am fine with adding another package, but I think there is a simpler solution that helps us override the default values: https://stackoverflow.com/a/71992310/9138914

So using rest operators won't work as it will completely replace the value for a given key. So if we have a default style for h1 and a custom style comes in for h1 then the whole default style is lost for h1. But we don't want that as we want the styles to be merged if present in both places.

I can replace it with my own code to compare and merge the styles if we don't want to add in another package but I believe this package will help us in the long run as it has many helper functions for working with collections and it's an official package by dart devs.

Here's an example

void main() {
  print({
    ...{
      'a': {'aa': 1, 'ab': 2},
      'b': {'ba': 3},
    },
    ...{'a': {'ac': 5}},
  });
}

The output will be: {a: {ac: 5}, b: {ba: 3}}