feat: implement design system

[arianeleonard]

GitHub Issue: #

Proposed Changes

• Bug fix
• Feature
• Code style update (formatting)
• Refactoring (no functional changes, no api changes)
• Build or CI related changes
• Documentation content changes
• Other, please describe:

Description

The application's design system was implemented, including color scheme and fonts.

Impact on version

• Major
  • The template structure was changed.
• Minor
  • New functionalities were added.
• Patch
  • A bug in behavior was fixed.
  • Documentation was changed.

PR Checklist

Always applicable

No matter your changes, these checks always apply.

• Your conventional commits are aligned with the Impact on version section.
• Updated CHANGELOG.md.
  • Use the latest Major.Minor.X header if you do a Patch change.
  • Create a new Major.Minor.X header if you do a Minor or Major change.
  • If you create a new header, it aligns with the Impact on version section and matches what is generated in the build pipeline.
• Documentation files were updated according with the changes.
  • Update README.md and src/cli/CLI.md if you made changes to templating.
  • Update AzurePipelines.md and src/app/README.md if you made changes to pipelines.
  • Update Diagnostics.md if you made changes to diagnostic tools.
  • Update Architecture.md and its diagrams if you made architecture decisions or if you introduced new recipes.
  • ...and so forth: Make sure you update the documentation files associated to the recipes you changed. Review the topics by looking at the content of the doc/ folder.
• Images about the template are referenced from the wiki and added as images in this git.
• TODO comments are hints for next steps for users of the template and not planned work.

Contextual

Based on your changes these checks may not apply.

• Automated tests for the changes have been added/updated.
• Tested on all relevant platforms

Other information

Internal Issue (If applicable):

Product Backlog Item 304365: Implement design system in Flutter Application Template