Change Flutter App Bar Background Color and text color

[nathanbaleeta]

Consultation with the U-Reporters testing team(WAGGGS) revealed they would prefer a silver app bar background and a contrasting dark text color(e.g black or dark blue)
Follow the steps below:

• Create a new branch from develop called change-appbar-background. Hint: git checkout -b <new-branch> <existing-branch>
• To change color of Application Bar in MaterialApp class, you can set the primarySwatch in theme or you can also change color of Application Bar using Scaffold by setting the backgroundColor in appBar of your Scaffold.
• Write a test for your improvement and add it to test/bottom_navigation_widget_test.dart
• Commit the changes (as a single commit).
• Push changes to github. Hint: git push --set-upstream origin change-appbar-background
• Open a pull-request.

Hint:
https://googleflutter.com/flutter-app-bar-color/#:~:text=of%20your%20Scaffold.-,A%20quick%20code%20snippet%20to%20change%20the%20app%20bar%20color,bar%20to%20Color(0xff885566)%20.

[nathanbaleeta]

@alee8599 your tests are failing. Run flutter test on your machine to verify your tests pass before opening another pull request.