[PLAY-1435] Fix Dark Mode Styles: Collapsible Kit

[anthonymig88]

What does this PR do?
In the Collapsible Kit:

• Under Custom Icons [React]: The icon wasn't visible on half the time (Looked correct in light mode)
• Under Custom Main [Rails/React]: The title had a white background with default text
• Under Custom Main with Icon [Rails/React]: The title had a background and the icon isn't showing up against the white background in dark mode
• Under useCollapsible Hook [React]: The nested icons were using light mode instead of dark mode

Screenshots: Screenshots to visualize your addition/change

How to test? Steps to confirm the desired behavior:

1. Go to Playbook --> Collapsible Kit
2. Click on Custom Icons dropdown to prove working
3. Scroll down to Custom Main and Custom Main with Icon to verify visual appeal in dark mode
4. Scroll down to Collapsible Hook to verify visual appeal in dark mode

Checklist:

• LABELS Add a label: enhancement, bug, improvement, new kit, deprecated, or breaking. See Changelog & Labels for details.
• DEPLOY I have added the milano label to show I'm ready for a review.
• TESTS I have added test coverage to my code.