- Pushes the borders to the .accordion-item
- Simplifies the .accordion-flush
- Recreates the .accordion-button border-bottom with an inset box-shadow
- Updates background-color to solid to match other components like list group and prevent stacked translucent borders
- Use negative margin to overlap the consecutive items
Switches to slightly more verbose, but more consistent, z-index layering we use elsewhere (e.g., pagination). Doing this ensures we're not toggling position on and off, but rather z-index.
