[Mobile] Flutter Quill Custom Style Builder does not work for 'N' header font

To reproduce:
Set custom style:
```
                customStyleBuilder: (a) =>
                    CupertinoTheme.of(context).textTheme.textStyle,
```
Use basic toolbar: 
`QuillToolbar.basic(controller: _controller),
`
Click 'N' on toolbar and notice the font color does not change.