lerp abstract method
Linearly interpolate with another ThemeExtension object.
t argument represents position on the timeline, with 0.0 meaning
that the interpolation has not started, returning
a (or something
a), 1.0 meaning that the interpolation has finished,
b (or something equivalent to
b), and values in between
meaning that the interpolation is at the relevant point on the timeline
b. The interpolation can be extrapolated beyond 0.0 and
1.0, so negative values and values greater than 1.0 are valid (and can
easily be generated by curves such as Curves.elasticInOut).
ThemeExtension<T> lerp(covariant ThemeExtension<T>? other, double t);