``Development``: Fix issue with Associations in UML Diagram (Dark Mode)

[minrows]

Checklist

• [x] I added multiple screenshots/screencasts of my UI changes

Motivation and Context

This PR fixes the issue with the color of associations in dark/light mode of Apollon. Composition Association is now displayed correctly. Issue Number: https://github.com/ls1intum/Apollon/issues/225

Steps for Testing

1. Make a composition association and aggregation association
2. Switch to dark/light mode
3. See the corrected form

Screenshots

Light Mode: Dark Mode: