Specifically, I think we need a version of ThemeData.copyWith that implements the logic of the main ThemeData constructor. There's no way right now to take a ThemeData, and say "ok now assume you had this swatch instead, what would you have to change".

We should probably use the same technique that I used in #9358.

Over 2 years later is this ever being implemented?

@dllz flutter has 7000+ issues, and this one is marked as severe so I'm guessing the implementation is non-trivial.

Would be nice to see it on the roadmap though

There is still an apparent bug that one cannot override the brightness for a specific scaffold as the Theme does not override. Eg.,

import 'package:entropy/util/extensions.dart';
import 'package:entropy/views/landing/landing.dart';
import 'package:flutter/material.dart';

class LandingWrapper extends StatelessWidget {
  const LandingWrapper({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Theme(
      data: ThemeData(brightness: Brightness.dark),
      child: Landing(),
    );
  }
}

Is there a fix to this?

There should be a merge method for all styling classes (*Decoration, *Style, *Theme, *ThemeData). This would make Theme.merge easier.

By the way, why are there so many terms to refer to styling? Is there a technical reason?