extensions property
final
      Arbitrary additions to this theme.
To define extensions, pass an Iterable containing one or more ThemeExtension subclasses to ThemeData.new or copyWith.
To obtain an extension, use extension.
This sample shows how to create and use a subclass of ThemeExtension that
defines two colors.
    
        link
    
    To create a local project with this code sample, run:
            flutter create --sample=material.ThemeData.extensions.1 mysample
        
See also:
- extension, a convenience function for obtaining a specific extension.
 
Implementation
final Map<Object, ThemeExtension<dynamic>> extensions;