title: Flutter theme not being overridden
body: <p>I've been trying to override my app theme for hours now without success, and I'd be very grateful for any help I can get.</p>
  
  <p>I think I'm doing exactly what <a href="https://stackoverflow.com/questions/52345077/cleanly-overriding-parts-of-a-theme-locally-in-flutter">this questioner</a> is doing, but for me the theme is not overridden for some reason and I can't understand why. No matter what I do the text 'hey' is not white. Any ideas why? Thank you.</p>
  
  <pre><code>@override
  Widget build(BuildContext context) {
    final ThemeData _themeData = Theme.of(context);
    return Theme(
      data: _themeData.copyWith(
        textTheme: _themeData.textTheme.copyWith(
          headline: _themeData.textTheme.headline.copyWith(
            color: Colors.white,
          ),
        ),
      ),
      child: Center(
        child: Text(
          'hey',
          style: Theme.of(context).textTheme.headline,
        ),
      ),
    );
  }
  </code></pre>