Add custom property to ThemeData in Flutter

Sadly, it appears that you simply can't - and the Flutter team doesn't seem to be interested in adding this, given their suggestion.

I think this is a major flaw, because we can't benefit from Theme.of(context) to automatically update all of our Widget that consume this ThemeData.

While some may say that you can use extensions to add new properties, you effectively won't know how to differ between multiple ThemeData (unless you can use some properties like Brightness, but I think this is just too hacky and not reliable to do so).

The alternative is to create another InheritedWidget (just like they said in the issue mentioned above) to handle your custom theme properties.

Edit: It seems that a new PR has introduced the possibility to extend the ThemeData, but it hasn't landed in main or even stable yet.

Flutter has recenty introduced ThemeExtensions (thanks @guilherme-matuella for the PR link!)

You can get a pretty good idea on how to use the functionality by following examples from the main Flutter repo:

return MaterialApp(
  title: MyApp._title,
  theme: ThemeData.light().copyWith(
    extensions: <ThemeExtension<dynamic>>[
      const MyColors(
        brandColor: Color(0xFF1E88E5),
        danger: Color(0xFFE53935),
      ),
    ],
  ),
  darkTheme: ThemeData.dark().copyWith(
    extensions: <ThemeExtension<dynamic>>[
      const MyColors(
        brandColor: Color(0xFF90CAF9),
        danger: Color(0xFFEF9A9A),
      ),
    ],
  ),
  themeMode: isLightTheme ? ThemeMode.light : ThemeMode.dark,
  home: Home(
    isLightTheme: isLightTheme,
    toggleTheme: toggleTheme,
  ),
);

Which you can later retrieve in your widget like so:
final MyColors myColors = Theme.of(context).extension<MyColors>();

Instead of adding custom property, we can extend ThemeData by extension function. For example, if we need a custom property for color, we can add extension function on ColorScheme. Color dependencies are now moved to Themedata.

// checking brightness to support dynamic themeing
extension CustomColorSchemeX on ColorScheme {
  Color get smallBoxColor1 =>
      brightness == Brightness.light ? Colors.blue : Colors.grey[400];
}

And then access that property through Theme.of(context)...

                        Container(
                          decoration: BoxDecoration(
                            border: Border.all(
                                color: Theme.of(context)
                                    .colorScheme
                                    .smallBoxColor1),
                        ),