Use case

ThemeExtension has been added as a way to support custom colors or theme properties (ref: #31522).
It forces you to implement lerp() method which is used to linearly interpolate between two ThemeExtension.
A lot of classes already define their own lerp function like Size.lerp, Color.lerp, RRect.lerp, Radius.lerp, etc.

A common way of defining related colors in custom themes is using a ColorSwatch. However, it does not define its own lerp() function at the moment, what makes implement ThemeExtension.lerp() function a difficult task.

Proposal

Define a static lerp() function for ColorSwatch:

static ColorSwatch<T>? lerp<T>(ColorSwatch<T>? a, ColorSwatch<T>? b, double t)

It could delegate in Color.lerp() for every color in the swatch table.