feat:define theme and fix widgets style

[rubuy-74]

Defines a theme for the application and fixes the style for the already existing widgets.

Closes #1404 [Description of the changes proposed in the pull request. Include steps to replicate the behavior and screenshots if UI is updated]

Review checklist

• [ ] Terms and conditions reflect the current change
• [ ] Contains enough appropriate tests
• [ ] If aimed at production, writes a new summary in whatsnew/whatsnew-pt-PT
• [ ] Properly adds an entry in changelog.md with the change
• [ ] If PR includes UI updates/additions, its description has screenshots
• [ ] Behavior is as expected
• [ ] Clean, well-structured code

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 29 lines in your changes missing coverage. Please review.

Project coverage is 12%. Comparing base (0d8b0af) to head (92e33f6). Report is 12 commits behind head on ui/redesign.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## ui/redesign #1405 +/- ## =========================================== - Coverage 12% 12% -0% =========================================== Files 266 265 -1 Lines 7201 7209 +8 =========================================== Hits 806 806 - Misses 6395 6403 +8 ```