added smooth dark mode transition

[mshokoya]

Pull Request

Related Issues: #274

Fixes #(issue number)

274

Description: added smooth transition from light-mode to dark-mode

Checklist:

• [x] I have tested my changes.
• [x] My code follows the project's coding standards.
• [ ] I have updated the documentation (if applicable).

Screenshots:

before link to 'before' video

after https://github.com/ani1609/Spendwise/assets/13862160/05406a40-cfdc-485b-a2b6-d74a541ed077

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