Add material_ui support

[Shakle]

Can't use theme from material_ui which is now official MarkdownStyleSheet.fromTheme(Theme.of(context))

[deakjahn]

@Shakle Yes, waiting. However, you can use a trick to make it compile in the meantime:

import 'package:flutter/material.dart' as legacy;

and using:

legacy.Theme.of(context)