WIP

[butschster]

1. Complete code refactoring
2. Covering with tests
3. Add dark theme for UI

Reason for This PR

[Author TODO: add issue # or explain reasoning.]

Description of Changes

[Author TODO: add description of changes.]

License Acceptance

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

PR Checklist

[Author TODO: Meet these criteria.]
[Reviewer TODO: Verify that these criteria are met. Request changes if not]

• All commits in this PR are signed (git commit -s).
• The reason for this PR is clearly provided (issue no. or explanation).
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this PR.
• Any user-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.