Flutter: Add custom color to existing theme - flutter

Question

I am using Flutters built-in themes like this:
return MaterialApp(
theme: ThemeData.light().copyWith(
primaryColor: const Color(0xFF5E975A),
bottomAppBarColor: const Color(0xff282828),
// ... And so on
),
As you can see I am modifying the existing theme with copyWith. Now let's say I want a certain button to always have the Color(0xFFFF0000). Is there a way to add a new key to the existing theme?
Something like this:
ThemeData.light().copyWith(
...
).addKey(myCustomColor: const Color(0xFFFF0000))
If not, what is the best-practice way to define my custom color? I feel like just declaring a global static variable is not the intended way to implement this.

Answer 1

Update for Flutter 3:
Flutter has answered this exact question in the newest version of Flutter. You can now create ThemeExtensions.
This can look something like this (after defining your ThemeExtension):
MaterialApp(
theme: ThemeData.light().copyWith(
extensions: <ThemeExtension<dynamic>>[
CustomColors.light,
],
),
darkTheme: ThemeData.dark().copyWith(
extensions: <ThemeExtension<dynamic>>[
CustomColors.dark,
],
),
// other parameters...
);
In the future there will be definitely more information about this but for now have a look at this article. I took the sample from there.

Answer 2

I am still finding the best practice way of defining ThemeData. For now, I have already used 2 ways to achieve the custom colors:
1 Use Extension
// use it with "Theme.of(context).myCustomColor"
extension CustomThemeDataExt on ThemeData {
Color get myCustomColor {
if(brightness == Brightness.light){
return Color(0xFFFF0000);
} else {
...
}
}
...
}
...
2 Define a custom theme
// use it with "CustomTheme.of(context).myCustomColor"
class CustomTheme {
final BuildContext context;
const CustomTheme(this.context);
static CustomTheme of(BuildContext context) => CustomTheme(context);
Color get myCustomColor {
if(Theme.of(context).brightness == Brightness.light){
return Color(0xFFFF0000);
} else {
...
}
}
}
Both need to import the relative file.