Best way to merge a tree of ThemeData.

[Solido]

ThemeData offers some merge methods but only for a single child.
I would like to be able to merge children

final mainThemeData = ...

MixTheme(
   data: mainThemeData.merge(subThemeData1, subThemeData2, ...)
)

It's a simple fold but I would like to check that I'm having the right approach when merging at the top ThemeData.

https://github.com/conceptadev/mix/pull/466/files