AdminLTE/scss/dark/bootstrap/_close.scss

.btn-close {
  color: $btn-close-color-alt;
  background: transparent escape-svg($btn-close-bg-alt) center / $btn-close-width auto no-repeat; // include transparent for button elements
  opacity: $btn-close-opacity-alt;

  // Override <a>'s hover style
  &:hover {
    color: $btn-close-color-alt;
    opacity: $btn-close-hover-opacity-alt;
  }

  &:focus {
    box-shadow: $btn-close-focus-shadow-alt;
    opacity: $btn-close-focus-opacity-alt;
  }

  &:disabled,
  &.disabled {
    opacity: $btn-close-disabled-opacity-alt;
  }
}

.btn-close-white {
  filter: $btn-close-white-filter-alt;
}