Custom ThemeExtension for Brand Tokens

Material 3 gives you a rich ColorScheme, but real products carry extra design decisions that don't map to any built-in slot: a brand gradient, a success color, a promotional accent, a custom card radius.

Hardcoding these as global constants breaks down the moment you support light and dark modes, because a single constant can't change with the active ThemeData.

• You want these values to live inside the theme.
• You want them to interpolate smoothly during theme animations.
• You want to read them with the same Theme.of(context) ergonomics as everything else.

Flutter's answer is ThemeExtension.

ThemeExtension<T> is an abstract class you subclass to attach your own typed bundle of values to a ThemeData.

The generic parameter T is your own class. This is what lets Flutter store and later retrieve your extension by its exact type instead of by a string key.

• It is type-safe: no casting from a Map.
• It is theme-aware: it ships inside ThemeData, so light and dark can hold different instances.
• It supports animation: Flutter calls lerp to blend two instances when themes change.

You must implement two methods: copyWith and lerp.