doc/dev/plugins/problems/scss/mixins/_shadow.scss

// Component focus shadow
@mixin control-shadow($color: $primary-color) {
  box-shadow: 0 0 0 .1rem rgba($color, .2);
}

// Shadow mixin
@mixin shadow-variant($offset) {
  box-shadow: 0 $offset ($offset + .05rem) * 2 rgba($dark-color, .3);
}