Add Z-Index to Scrollbar Rail Wrappers

[mariobehling]

Objective:
Enhance the visibility and layering of scrollbar rail wrappers by adding a z-index property.

Details:
To ensure that the scrollbar rail wrappers are correctly layered and not obstructed by other elements, we need to add a z-index property.

Tasks:

1. Update the CSS for .scroll-parent to include the z-index property for .bunt-scrollbar-rail-wrapper-x and .bunt-scrollbar-rail-wrapper-y.
2. Test the changes to ensure the scrollbar rail wrappers are properly layered and visible.

Why This Matters:
Adding a z-index to the scrollbar rail wrappers will improve the user interface by ensuring that these elements are not hidden behind other content. This change will enhance the usability and visual consistency of the application.