fix border issue for nested card inside accordions

Created by: ovpv

After analysing the code in scss/_card.scss, in line 281, the styles added for .card class inside .accordian class is targeting all the cards inside instead of its immediate children elements with the class .card.

Code now:

.accordion {
  .card {
    ...
  }
}

Fix:

.accordion {
  > .card {
    ...
  }
}

Fixes #27510 (closed), fixes #26556 (closed).