(Urgent) BugFix: Fixed accentColor and backgroundColor deprecation and added null safety to example. Works on Flutter 3.10

[varun-2001]

Detailed Changes:

1. Theme.of(context).backgroundColor => Theme.of(context).colorScheme.background
2. Theme.of(context).accentColor=> Theme.of(context).colorScheme.accent
3. FlatButton => TextButton
4. Colors.grey[shade300] => Colors.grey.shade300

[Pablo-Aldana]

Hi! Seeing the urgency of the issue, I have created a fork with the fixed and uploaded a new package to pub.dev in case you'd like to use it.

Liquid Progress Indicator Dart3 compatible