Setup a theme / styles for the app

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Match colors, type, and more to look similar to the Umami website demo in Drupal 8.x.x. https://www.drupal.org/docs/8/umami-drupal-8-demonstration-installation-profile

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Helpful links:

• https://link.medium.com/fgRkVgrYM6
• https://medium.com/flutter-community/themes-in-flutter-part-1-75f52f2334ea
• https://medium.com/flutter-community/themes-in-flutter-part-2-706382bc32c5
• https://medium.com/flutter-community/themes-in-flutter-part-3-71361ffdc344